In this work, a macromolecular coupling agent (BA-MAA-AN tercopolymer) was used for surface modification of native nano-sized silicon nitride (Si3N4) powder. This modification strategy was designed for preparing nano-Si3N4/NBR composites. The structure and surface properties of modified nano-Si3N4 were systematically investigated by FTIR, XPS, TGA, TEM, Size Distributions Analyzer, and Contact Angle Measurement. It was found that, the optimum loading of BA-MAA-AN tercopolymer coated on the surface of nano-sized Si3N4 is 10% of nano-Si3N4. According to the spectra of FTIR, XPS and TGA, it can be inferred that this macromolecular coupling agent covalently bonds on the surface of nano-sized Si3N4 particles and an organic coating layer is formed. The contact angle experiments show that the hydrophobic property of nanosized Si3N4 modified with macromolecular coupling agent is improved obviously. TEM reveals that modified nano-Si3N4 possesses good dispersibility and the average diameter in NBR is less than 100 nm. It has also been found that the oil resistance of NBR based nanocomposites is improved greatly due to the modified nano-Si3N4. (C) 2007 Wiley Periodicals, Inc.