How i make transparent form with one button ?

It would be a pretty weird thing, but…

You can do it, but not via transparency. You can (say, temporarily) make a form's Region a small rectangle (or even non-rectangular region) around your button. It won't resize the form. Not only cut out parts of the form will be transparent, they will be transparent even for mouse, so the form will behave as a fragment of a form, with all of the rest cut out. This is the method of making non-rectangular forms, by the way, and the form can be quite complicated, as you can logically combine regions.

All you need is this: http://msdn.microsoft.com/en-us/library/system.windows.forms.control.region.aspx[^].

Create some desired region object and assign it the the property Region.

—SA