I'm brand new to these forums, but I am doing a little work with some JInternalFrames myself, and I have a set-up very similar to yours. I am using a JDesktopPane with menus as my "MainWindowFrame". When a user click the File menu and selects New..., it opens an Internal Frame created by an instance of a separate class file that extends JInternalFrame. To get the JInternalFrame to open maximized, use this method immediately after construction: