Got it!
in the global variables create delegate and reference for form B as follow:
Private Delegate Sub del()
Dim slice_SendingStat As B = New B()
creating a function in A as follow:
Private Sub PauseTimer()
slice_SendingStat.TimerEffects.Enabled = False
End Sub
in the btnSend_click function, put this:
Me.Invoke(New del(AddressOf PauseTimer))
that will get access to the form B delegate and pause the timer!!!