The generalized kinematic equation for condensate film thickness is taken into consideration at the liquid-vapor interface and is used to investigate the nonlinear stability of film flow down a vertical wall in an applied transverse uniform magnetic field. Results show that both the supercritical stability and the subcritical instability may occur in film flow. The results also indicate that the flow would be stabilized if the mass was transferred into the liquid phase. The supercritical filtered waves are always linearly stable with respect to side-band disturbance. The effect of the magnetic field which can be revealed as the Hartmann number, m, is to stabilize the flow. Therefore, the instability could be counteracted by controlling the applied magnetic held.