By the nonlinear formulation power enhancement is treated by a tapered wiggler in a two-stream Free Electron Laser (FEL. For small axial magnetic field and Group I trajectories the wiggler field was tapered downward in order to achieve an efficiency enhancement. Wave-particle interaction is described by a set of nonlinear coupled differential equations in 1D approximation. The scaled vector potential correspond to the out-put power is plotted versus scaled axial distance. It is found, the output power may be enhanced to a level of almost 99.2%.