Abstract: Chemical vapor infiltration of propylene into C/C composites was studied by numeric simulation and an improved model for homogeneous gas-phase reactions of propylene pyrolysis was proposed. The model consists of 34 elementary reactions of 16 species. Then a bipore model for describing the changes of the pore topology with the progress of densification and a computer code for simulation of surface reactions by other authors were coupled together to predict the process. The numerical simulation showed that the density distribution of C/C composites is dependent on residence time of gas, densification temperature and densification time. There are two infiltration stages (rapid densification of micro-pores and successive densification of macro-pores) which act closely with the concentration distributions of main homogeneous pyrolytic products of propylene. Among them the concentration distribution of benzene seems most apparently to influence the densification speed and C/C density uniformity. The infiltration would be terminated when the benzene rich region becomes narrow, and the surface crusting of C/C appears. The present model is validated by comparing the predicted densities with the experimental ones.