Abstract: Birds deploy several short feathers called alula at the leading edge of wings to suppress stall and increase lift. The sweep angle between the alula and the wing is adjusted according to different flight states. The effects of the alula sweep angle on the aerodynamic performance of the rectangular wing are therefore investigated by utilizing force measurement and particle image velocimetry. Compared to an alula with a zero sweep angle, an alula with an appropriate non-zero forward sweep angle can increase the lift and does not introduce extra drag, thus improves considerably the aerodynamic performance. The underline mechanism is revealed by the flow fields measured by planar and stereo particle image velocimetries, which demonstrate that the alula with an appropriate non-zero forward sweep angle enhances the strength of the leading edge vortex over the wing and broadens the range of angle of attack in which the leading edge vortex can increase the lift.