The instability of field-aligned Whistler waves is studied for subtracted bi-Maxwellian magneto- Plasma in the presence of perpendicular AC electric field by using the method of characteristic solutions and kinetic approach. The dispersion relation has been derived. The growth rate has been calculated for magnetospheric Plasma. Calculation shows that either a loss- cone or a thermal anisotropy in the hot plasma component of the magnetosphere can lead to the generation of incoherent emission of low frequency Whistler waves. The effective parameters for the generation of Whistler mode wave are not only the temperature anisotropy but also the relativistic factor, AC field frequency, amplitude of substracted distribution and width of the loss-cone distribution function which are discuss in result and discussion section.