Abstract: Natural rubber (NR) reinforced with pitch-based chopped carbon fiber (PCF) modified by nitric acid (HNO₃) or blending with carbon black (CB) or Silica was studied. The effects of the surface modification and blending on the surface morphology and microstructure of PCF and mechanical properties of NR matrix composites were investigated, and the mechanism of interfacial interaction between the filler and the matrix was analyzed. The results show that the surface roughness and O/C ratio of PCF are improved evidently, while graphitization degree decrease somewhat by HNO₃ treatment. PCF has no reinforcing effect on NR when used alone. However, the tensile strength of NR improves martially by blending PCF with CB or Silica, which enhance from about 3 MPa to 18.6 MPa and 23.7 MPa, respectively, and to 24.5 MPa and 24.7 MPa after modification. Compared with of CB/NR vulcanizates, the tensile strength of PCF-CB/NR vulcanizates decreases slightly, but it enhances 25.6% for PCF-CB/NR vulcanizates. The tensile strength of PCF-Silica/NR where PCF modified or not is a little lower modifieated than that of Silica/NR vulcanizates. Combined with mechanical properties, 50% modulus of different reinforcing system and SEM of fracture section, it can be deduced that the improved interfacial interaction in PCF-CB and NR could be mainly attributed to adsorption theory and a subtle mixture of mechanical interlocking and adsorption theory before and after modification of PCF, but nearly hydrogen bonding in PCF-Silica and NR interface, whether modified or not.