@aria_aa said in how too add an exciting ".ui" file to a "cpp widget application project"?: What should I do? clean up the build directory run qmake rebuild project Should work, code looks good. And another question: what is #include "ui_window_no_000.h" and #include "ui_window_no_001.h" in cpp files? there is no file such as ui_window_no_000.h or ui_window_no_001.h in project. These files are automatically generated from your .ui files by uic. It's part of the build process in Qt, same as with it running moc to generate meta-object functionality.