In frequency modulation system the change of the percentage modulation affects the bandwidth of the transmitted signal. Outside the critical sideband the amplitude of all the higher order sidebands is a positive decreasing function of the order of the sidebands. In this paper we give an approximate determination of amplitude of this critical sideband as given by equation (8) or fig. (2); given also are the approximate magnitude of any side band above the critical and the ratio of the amplitude of any sideband above the critical to that of the critical, equation (11). By means of these two equations we can design the bandwidth of the transmission system by retaining a certain number of significant sidebands above the critical.