mdi form become like shadow

If this is an MDI solution, then that isn't really what you do - the MDI parent is a container for all it's child forms, so if you dim it out and disable it, then that same will happen to it's children.

If what you are trying to do is to display a form in place of the MDI, then use the Hide method on the MDI form, and use a non-MDI child form as the one you call ShowDialog on. Provided you Hide first, then ShowDialog, it should work.