Porous anodic aluminum oxide (AAO) membranes were sputtered by 500eV argon ion ( Ar+) beam. Scanning electron microscopy and atomic force microscopy i mages show that not only the barrier layer of AAO can be easily removed by Ar+ beam sputtering, but also several categories of surface morphologies w ere got by changing the incidence parameters. For instance, off-normal incidence bombardment results in ripple formation on the surface of AAO and off-normal in cidence during smple stage rotation leads to AAO surface smoothing. Theory of ri pple formation and evolution on amorphous substrate under ion beam irradiation d eveloped by Bradley and Harper is used to interpret the characteristics of rippl es on AAO surface.