Arrays of Co nanowires were fabricated by ac electrochemical deposition of metal into high order anodic alumina templates prepared through two-step oxidation method. The effects of applied magnetic field and the pH value of the electrolyte on growth of nanowires were investigated. Arrays of Co nanowires of hcp structure were deposited in the electrolyte whose pH values are 6.0 and 6.5 without magnetic field, then 0.3 T magnetic field was applied along the long axis of nanowires. The experimental results show that external magnetic field and adjustment of pH value of the electrolyte impel the easy magnetization axis of the hcp Co grain to grow along length direction of nanowires. The Co nanowire arrays show higher perpendicular anisotropy, larger coercivity and high M/Ms because the superposition of the crystal anisotropy of hcp Co grains and shape anisotropy of nanowire along length direction.