Abstract: Electroless plating Ni-P composite coating reinforced with nano-SiC particles on the 45# steel substrate was prepared by adding different contents of nano-SiC particles in an optimized electroless bath. The microstructure, morphology, microhardness, and formation mechanism of the samples were analyzed using micro-hardness tester, scanning electron microscopy and X-ray diffraction. The results showed that the coatings of Ni-P, Ni-P-SiC are amorphous and the hardness values are 572 HV and 649 HV, respectively. The coatings are converted from amorphous to crystalline by heat treatment and the hardness increases obviously. The maximum hardness of the Ni-P coating and the Ni-P-nano-SiC composite coating is 1045 HV and 1341 HV after heat treatment at 400°C for 1 h. Nano-SiC particles do not take chemical action in electroless bath. They only are deposited in composite coating with the Ni and P and play a particle composite reinforcing role. The Ni-P-nano-SiC composite coating grows in layer mode. Its growth direction is perpendicular to the surface of the steel substrate. Nano-SiC particles can increase the speed of the electroless composite plating and improve the growth of the composite coating with a thin sub-layer mode.