Hardness of electronless Ni-P plating with 10.98wt.% P content was investigated. Heat treatment temperature and time of the plating was optimized by Uniform Design method. Structure of the plating was analyzed by XRD method. The results show that correlation consist between maximum hardness of the Ni-P plating and heat treatment temperature, not heat treatment time under the experimental condition, and max. of the plating hardness appears when the heat treatment is about 430°C . With increase of heat treatment temperature, structure of the plating transitioned gradually from amorphous state to crystalline state, and Ni3P crystal grain grown up, and it affect hardness of the plating.