We have investigated the magnetization properties of Co(P) nanowire arrays fabricated by ac electroforming into self-ordered nanosized pores in anodic aluminum oxide (AAO), which was prepared by anodizing the pure aluminum foil. The nanopores were found to have 20 nm diameter and aspect ratios of more than several hundreds. According to the magnetic properties of Co(P) nanowire prepared, it was found to have the coercivity of more than 2.5 kOe due to the shape anisotropy and squareness(M-r/M-s) of similar to0.8. It was particularly interesting to note that Co(P) nanowire showed different crystallographic structures depending on the applied potential. Co(P) nanowire prepared at ac 10 V-rms exhibited better magnetic properties than one prepared at ac 11 V-rms due to the crystallographic structural variation.