Abstract: Effect of concentration and size of SiC particle in plating bath on the structure and hardness of electroless Ni-P-SiC plated composite coats was systematically studied.The test results showed that the precipitating amount of SiC in the composite coats increases with concentration of SiC in the plating bath and reaches the maximum at the size of SiC about 7.0μm when the concentratin is certain. The concentration and particle size of SiC in the plating bath have little effect on the hardness of as received composite coats, but they obviously affect the hardness of the composite coats heat treated at 400℃.