We have investigated the dimensional and microstructural dependence of magnetic proper-ties of Co(P) nanowire arrays fabricated by ac/dc electroforming into different size nanopores in anodic aluminum oxide (AAO). Our results indicate that the magnetic properties of nanowire arrays can be varied with their dimensions and microstructures. As for the Co(P) nanowire arrays with the diameter of 20 nm, it was found to have the coercivity of 2.58 kOe due to the shape anisotropy and squareness(M-r/M-s) of 0.78. The Co(P) nanowire arrays with the diameter of 200 nm, however, showed very different magnetic properties depending on the current densities. Nanowires fabricated at relatively low current density had the tendency to have the preferred crystallographic orientation of (0002) parallel to the nanowire. These microstructural differences are the reason why Co(P) nanowire arrays prepared at different current densities exhibited different magnetic properties.