Abstract: In situ synthesized TiB/ TiN-Ti base composite coatings were fabricated on Ti-3Al-2V titanium alloy surface with different BN particle sizes and Ti powders by laser cladding technology.Scanning electron microanalysis (SEM), X-ray diffraction(XRD) and microelement distribution(EPMA) were used to analyze the phase morphology, phase composition and element distribution. The reaction mechanism of in-situ synthesis of Ti-BN system under laser cladding condition was discussed. The results show that TiB/TiN-Ti composite coating is prepared with different BN particle sizes and Ti powders.The situ reaction mechanism of coarse BN is mainly controlled by diffusion mechanism and BN is formed to promote the nuclear growth.The in situ reaction mechanism of fine BN is mainly controlled by the dissolution-precipitation mechanism.