Something like this may Help :-
private void DataGridView1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
{
if (DataGridView1.CurrentCell.ColumnIndex == yourComboBoxColum)
{
ComboBox combo = e.Control as ComboBox;
if (combo == null)
return;
combo.DropDownStyle = ComboBoxStyle.DropDown;
}
}