Abstract: Using nitrogen, hydrogen respectively as the carrier gas, the C/C composite was fabricated by the thermal gradient CVI process under the condition of 1000℃,0.5 kPa propylene partial pressure. The influence of the carrier gas especially hydrogen on the structure of pyrocarbon was researched. Extinction angles of CVI pyrocarbon at different areas of the samples were tested by polarized light microscopy, which were used to analyze the alteration of the microstructure of pyrocarbon. The results show that the addition of nitrogen can not change the microstructure of pyrocarbon which plays an inert role. And the addition of hydrogen has more advantage to obtain rough laminar microstructure pyrocarbon by changing components of the micro-environment during CVI process. The influence of density addition on residence time of the reactants leads to an abrupt change of pyrocarbon microstructure. The better control of the structure of the pyrocarbon in the C/C composite can be achieved by adding hydrogen.