I am using C# 2005. How to show a child form fix into a main form which panel alocated when I click a button in main form. Thanks!

If you are referring to placing another form into a Panel on the Main form, this should really be done using a UserControl. However,

Form2 fm2 = new Form2();
            fm2.TopLevel = false;
            fm2.Dock = DockStyle.Fill;

You can put a form into another control (such as a panel) on a TopLevel form like your main form by using the code above. You should typically set the second form's control box off, and set the Text of the second form to empty, also set the border to none.. But that is just you choice.