Thank you for reply me first. I perfect sure this rows must selected.
1.Becuase my datagridview 's selection mode property selected FullRowSelect
2.If i use one form and test it that ok .it can run all the code and detect the dataGridView selected rows.

So i don't know why i selected rows in main form, after click the button show the option form and finish the checkbox check option ,
after run to main function "dataGridView selected rows", it can't detect selected rows. Thank you