The aerodynamics force generation of a butterfly (Morpho peleides) in forward flight was studied, using the method of numerically solving the Navier-Stokes equations in moving overset grids. The results shows that the vertical and thrust forces of the wings, which balances the weight and the body drag, are mainly contributed by the drag of the wings. During the downstroke, a strong ‘vortex ring’, consistes of the leading edge vortex, the wing tip vortices and thestarting vortex, was generated. The ‘vortex ring’ containes a jet parallel to the wing flapping direction, and the jet produces a large instantaneous drag. The vertical force is mainly due to the drag of the downstroke. The thrust force is mainly due to the drag of the wings in the upstroke.