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An immersed-boundary method is used to investigate the flapping wings with different aspect ratios ranging from 1 to 5. The numerical results on wake structures and the performance of the propulsion are given. Unlike the case of the two-dimensional flapping foil, the wing-tip vortices appear for the flow past a three-dimensional flapping wing, which makes the wake vortex structures much different. The results show that the leading edge vortex merges into the trailing edge vortex, connects with the wing tip vortices and then sheds from the wing. A vortex ring forms in the wake, and exhibits different patts for different foil aspect ratios. Analysis of hydrodynamic performances shows that both thrust coefficient and efficiency of the flapping wing increase with increasing aspect ratio.