Abstract: Polypropylene ( PP) composites with polybutyl acrylate (PBA) in-si tu graf ted nano-silica were preparedby a conventional melt compounding technique. The mechanism of the graf t polymerization and the st ructure of thenanocomposites were studied by means of Haake torque rheometer , inf rared spect roscopy , thermogravimet ry , X-rayphotoelect ron spect roscopy , transmission elect ron microscopy , etc. It is proved that PBA has been chemically bonded to the surface of nano-silica during the in2si tu melt graf t polymerization. The modified nano-silica particles arehomogeneously dispersed in PP mat rix and a st rong interfacial interaction is established by the chain entanglementbetween the graf ted polymer and the mat rix.