Abstract: To investigate the effect of spanwise flexibility on a flapping thrust and the reason for the effect, a flapping wing model with spanwise wing flexure is selected. Both Navier-Stokes equations and rigid motion equations are solved with respect to different spanwise elastic constants. The results indicate that a degree of spanwise flexibility is found to yield a decrease in the drag coefficient. But introducing a far greater degree of spanwise flexibility is found to be detrimental. The reason for the effect of spanwise flexibility on drag coefficients is that introducing spanwise flexibility changes the distribution of effective angles along the spanwise direction. If introducing spanwise flexibility could increase the absolute magnitude of an effective angle along the span, then the instantaneous drag coefficient can be decreased.