Two modified pitches with softening points of 70 degrees C (LPA) and 110 degrees C (HPA) produced from modification of naphthenic vacuum residue were cocarbonized with polystyrene (PS), respectively, and the mesophase pitches were obtained. The effects of reaction time, PS addition, pressure, and properties of raw materials on cocarbonization behavior were analyzed by H/C, quinoline insoluble content (QI), carbon residue value, density, polarized optical images, X-ray diffraction pattern (XRD), scanning electron microscopy (SEM) analysis, Fourier transform infrared (FTIR) spectra, and H-1 nuclear magnetic resonance NMR) analysis. Comparing with HPA, the cocarbonization process showed more suitable for LPA to prepare the mesophase pitch due to the favorable occurrence of radical chain transfer reactions between LPA and PS components which contributed to the existence of aliphatic side-chains and naphthenic structures in mesogens as well as the formation of a well-developed mesophase pitch. Mesophase pitch with larger domains optical texture and better crystal structure was obtained from cocarbonization with 5 wt % PS under the optimal conditions of 410 degrees C, 6 h, and 6 MPa.