AAO films with a novel six-membered ring structure were prepared by constant current anodization. The six-membered ring contains smaller pores, formed at the triple junction sites of the larger pores. The interpore distance was reduced from the usual 85 nm to 50 nm, doubling the usual pore density. These novel AAO film structures with high pore density may have applications in the preparation of mosaic nanowire arrays and photonic devices.