Abstract: From the angle of molecular design，a new macromolecular superficial modifier (LMPB-g-MAH) was synthesized by graft copolymerization of maleic anhydride （MAH）onto the long molecular chain of low-molecular-weight liquid polybutadiene rubber (LMPB)，and it was used as modifier for surface modification of nano-sized silicon nitride (Si↓3N↓4).The prepared LMPB-g-MAH as well as modified and unmodified nano-sized Si↓3N↓4 were characterized by FT-IR，TEM，TGA，particle size analysis and sedimentation test etc..Results showed that MAH has been grafted onto the long molecular chain of low-molecular-weight LMPB；while the graft degree，dosage，temperature and reaction time of the macromolecular superficial modifier are 9%~11%，10%~12%，65 ℃ and 3 h respectively，the particle size of nano-sized Si↓3N↓4 was reduced and the agglomeration of nano-particles was effectively restricted after surface modification.