Numerical simulations are conducted to investigate the mechanism of hovering flight with an inclined stroke. The Reynolds numbers considered are 150 and 1000 based on the maximum translational velocity and wing chord length. Three mechanisms responsible for high vertical force generation, suggested by Dickinson et al. (1999), are confirmed and more elaborated in the present study. First, we show that the vertical force during downstroke is larger than that from the quasi-steady analysis due to the delayed stall mechanism. Second, the wing-wake interaction of reducing the negative vertical force during the stroke reversal is explained in terms of the reattachment of the vortex, shed previously during downstroke, on the wing, by which the wing is submerged in a low pressure region during up stroke and has a smaller negative vertical force. Finally, the rotational circulation is explained by advancing the rotation timing of the wing at supination using both the numerical simulation and inviscid potential theory.