Highly ordered porous anodic alumina(PAA) films are fabricated with the sulfuric and oxalic acid as electrolyte and by the two-step anodization method. The crystal structure of aluminum foil before and after annealing process is examined with X-ray diffraction(XRD). Field-emission scanning electron microscope(FE-SEM) and atom force microscope(AFM) are used to observe the surface and cross morphology of PAA films. The pore diameters(20～100 nm), pore distances (50～150 nm), depths(0.2～70 μm) and densities 109～1012 /cm2 can be well controlled. The forming process and mechanism of PAA film are discussed. The effects of aluminium pre-treatments, the electrolytes type, anodizing voltages and electrolyte concentrations on PAA film′s stractures are analyzed.