diff --git a/app/views/book.py b/app/views/book.py
index 4811969..8db9fb4 100644
--- a/app/views/book.py
+++ b/app/views/book.py
@@ -332,19 +332,42 @@ def edit_contributor_role(book_id: int):
         return redirect(url_for("book.settings", book_id=book_id))
 
 
-#################
-# Collection CRUD
-#################
+###############################
+# Collection/SubCollection CRUD
+###############################
 
 
 @bp.route("/<int:book_id>/create_collection", methods=["POST"])
+@bp.route("/<int:book_id>/<int:collection_id>/create_sub_collection", methods=["POST"])
 @login_required
-def collection_create(book_id: int):
+def collection_create(book_id: int, collection_id: int | None = None):
     book: m.Book = db.session.get(m.Book, book_id)
     if not book or book.owner != current_user or book.is_deleted:
         log(log.INFO, "User: [%s] is not owner of book: [%s]", current_user, book)
         flash("You are not owner of this book!", "danger")
         return redirect(url_for("book.my_books"))
+    if collection_id:
+        collection: m.Collection = db.session.get(m.Collection, collection_id)
+        if not collection or collection.is_deleted:
+            log(log.WARNING, "Collection with id [%s] not found", collection_id)
+            flash("Collection not found", "danger")
+            return redirect(url_for("book.collection_view", book_id=book_id))
+        elif collection.is_leaf:
+            log(log.WARNING, "Collection with id [%s] is leaf", collection_id)
+            flash("You can't create subcollection for this collection", "danger")
+            return redirect(
+                url_for(
+                    "book.sub_collection_view",
+                    book_id=book_id,
+                    collection_id=collection_id,
+                )
+            )
+
+    redirect_url = url_for("book.collection_view", book_id=book_id)
+    if collection_id:
+        redirect_url = url_for(
+            "book.sub_collection_view", book_id=book_id, collection_id=collection_id
+        )
 
     form = f.CreateCollectionForm()
 
@@ -363,23 +386,27 @@ def collection_create(book_id: int):
                 label,
             )
             flash("Collection label must be unique!", "danger")
-            return redirect(url_for("book.collection_view", book_id=book_id))
+            return redirect(redirect_url)
 
         collection: m.Collection = m.Collection(
             label=label, about=form.about.data, version_id=book.versions[-1].id
         )
+        if collection_id:
+            collection.parrent_id = collection_id
+            collection.is_leaf = True
+
         log(log.INFO, "Create collection [%s]. Book: [%s]", collection, book.id)
         collection.save()
 
         flash("Success!", "success")
-        return redirect(url_for("book.collection_view", book_id=book_id))
+        return redirect(redirect_url)
     else:
-        log(log.ERROR, "Book create errors: [%s]", form.errors)
+        log(log.ERROR, "Collection/Subcollection create errors: [%s]", form.errors)
         for field, errors in form.errors.items():
             field_label = form._fields[field].label.text
             for error in errors:
                 flash(error.replace("Field", field_label), "danger")
-        return redirect(url_for("book.settings", book_id=book_id))
+        return redirect(redirect_url)
 
 
 @bp.route("/<int:book_id>/<int:collection_id>/edit", methods=["POST"])
diff --git a/tests/test_book.py b/tests/test_book.py
index ebbb0c9..73bf6a1 100644
--- a/tests/test_book.py
+++ b/tests/test_book.py
@@ -283,3 +283,128 @@ def test_crud_collection(client: FlaskClient, runner: FlaskCliRunner):
 
     assert response.status_code == 200
     assert b"Collection not found" in response.data
+
+
+def test_crud_subcollection(client: FlaskClient, runner: FlaskCliRunner):
+    _, user = login(client)
+    user: m.User
+
+    # add dummmy data
+    runner.invoke(args=["db-populate"])
+
+    book: m.Book = db.session.get(m.Book, 1)
+    book.user_id = user.id
+    book.save()
+
+    leaf_collection: m.Collection = m.Collection(
+        label="Test Leaf Collection #1 Label",
+        version_id=book.versions[-1].id,
+        is_leaf=True,
+    ).save()
+    collection: m.Collection = m.Collection(
+        label="Test Collection #1 Label", version_id=book.versions[-1].id
+    ).save()
+
+    response: Response = client.post(
+        f"/book/{book.id}/{leaf_collection.id}/create_sub_collection",
+        data=dict(
+            label="Test SubCollection #1 Label", about="Test SubCollection #1 About"
+        ),
+        follow_redirects=True,
+    )
+
+    assert response.status_code == 200
+    assert b"You can't create subcollection for this collection" in response.data
+
+    response: Response = client.post(
+        f"/book/{book.id}/{collection.id}/create_sub_collection",
+        data=dict(
+            label="Test SubCollection #1 Label", about="Test SubCollection #1 About"
+        ),
+        follow_redirects=True,
+    )
+
+    assert response.status_code == 200
+    assert b"Success!" in response.data
+
+    response: Response = client.post(
+        f"/book/{book.id}/{collection.id}/create_sub_collection",
+        data=dict(label="Test Collection #1 Label", about="Test Collection #1 About"),
+        follow_redirects=True,
+    )
+
+    assert response.status_code == 200
+    assert b"Collection label must be unique!" in response.data
+
+    sub_collection: m.Collection = m.Collection.query.filter_by(
+        label="Test SubCollection #1 Label"
+    ).first()
+    assert sub_collection
+    assert sub_collection.is_leaf
+    assert sub_collection.parrent_id == collection.id
+
+    # m.Collection(
+    #     label="Test Collection #2 Label", version_id=collection.version_id
+    # ).save()
+
+    # response: Response = client.post(
+    #     f"/book/{book.id}/{collection.id}/edit",
+    #     data=dict(
+    #         label="Test Collection #2 Label",
+    #     ),
+    #     follow_redirects=True,
+    # )
+
+    # assert response.status_code == 200
+    # assert b"Collection label must be unique!" in response.data
+
+    # new_label = "Test Collection #1 Label(edited)"
+    # new_about = "Test Collection #1 About(edited)"
+
+    # response: Response = client.post(
+    #     f"/book/{book.id}/{collection.id}/edit",
+    #     data=dict(
+    #         label=new_label,
+    #         about=new_about,
+    #     ),
+    #     follow_redirects=True,
+    # )
+
+    # assert response.status_code == 200
+    # assert b"Success!" in response.data
+
+    # edited_collection: m.Collection = m.Collection.query.filter_by(
+    #     label=new_label, about=new_about
+    # ).first()
+    # assert edited_collection
+
+    # response: Response = client.post(
+    #     f"/book/{book.id}/0/edit",
+    #     data=dict(
+    #         label=new_label,
+    #         about=new_about,
+    #     ),
+    #     follow_redirects=True,
+    # )
+
+    # assert response.status_code == 200
+    # assert b"Collection not found" in response.data
+
+    # response: Response = client.post(
+    #     f"/book/{book.id}/{collection.id}/delete",
+    #     follow_redirects=True,
+    # )
+
+    # assert response.status_code == 200
+    # assert b"Success!" in response.data
+
+    # deleted_collection: m.Collection = db.session.get(m.Collection, collection.id)
+    # assert deleted_collection.is_deleted
+
+    # response: Response = client.post(
+    #     f"/book/{book.id}/{collection.id}/delete",
+    #     follow_redirects=True,
+    # )
+
+    # assert response.status_code == 200
+    # assert b"Collection not found" in response.data