Hard to tell from your description what the problem is. General rule
is: You should only ever touch controls from the thread that creates
them. If you create the modal dialog in the main thread, you should
ShowDialog it from the main thread, using something like
MainForm.Invoke().

hi Sonali,
from your background thread, you should call a method using BeginInvoke to
transfer execution of that method to the main UI thread.  then you call
ShowDialog from that method and it will block the UI thread only, and the
modal function will work normally.
example below (untested):

private void MyAsyncMethod()
{
   // do stuff.. running on background thread
   // ...

   // ok, now want to ShowDialog on the main UI thread
   BeginInvoke(new MyDelegate(this.MethodForShowDialog), null, null);

   // the background thread will continue running asynchronously,
regardless of ShowDialog() on the main UI thread
}

delegate void MyDelegate();
private void MethodForShowDialog()
{
       // now running on UI thread
       MessageBox.ShowDialog(...whatever...);
}

is this what you want?
tim

--------------------------
blog: http://tim.mackey.ie