( Ni-Co ) / nano-Al₂O₃composite coatings were prepared by high frequency pulse electrodeposition. The effect of duty cycle on deposition rate, composition, morphology and surface microhardness of the ( Ni-Co) / nano-Al₂O₃composite coatings were investigated. The results indicate that the deposition rate and the size of composite coatings grains increases, the surface morphology becomes coarse, the content of Co reduces, that of Ni increases, and that of nanometer Al₂O₃particles is not obvious with the increasing of the duty cycle from 0 . 3 to 0 . 5 . The mircohardness decreases due to the abatement of Co.