The densification rate of C/C composites fabricated by directional flow thermal gradient chemical vapor infiltration process from C3H6, C3H6-N2 and C3 H6-H2 was investigated respectively. The mechanism on the role of carrier gas in chemical vapor infiltration was also discussed. The results shows that whether or not adding N2 as carrier gas has little influences on the densification behavior of C/C composites with the controlled temperature, the partial pressure of hydrocarbon and the effective residence time of the gas phase remain constant. When the controlled temperature is not less than 1 173 K, using N2 or H2 as carrier gas makes pronounced differences in densifying of C/C composites. The average bulk density of C/C composites from C3 H6-H2 is eight to ten percent higher than that from C3H6-N2. However, when the controlled temperature is not higher than 1 123 K,the densification rate of C/C composites from C3 H6-H2 is much lower than that from C3 H6-N2, which implies that effects of carrier gas on densification of C/C composites are closely related to the type of carrier gas and infiltration temperature. At higher temperature, using H2 as carrier gas is favorable to the densification of C/C composites,while at lower temperature, hydrogen, acting as reactive gas, can inhibit the formation of pyrolytic carbon.