Oxidation protective Si-Mo coatings were prepared by pack cementation on the surface of C/SiC coated carbon/carbon composites. The influences of Mo/Si mass ratio in the pack powders on the microstructure and oxidation resistance of the coating were investigated. It showed that with the Mo/Si ratio increasing from 0.1 to 0.4, the MoSi_2 proportion and the dimension of cracks in the coating increased, and the oxidation protective ability of the coated specimens firstly increased and then decreased. The coating with the Mo/Si of 0.2 exhibited better oxidation protective ability because of its more suitable MoSi_2 proportion and excellent microstructure than those with the Mo/Si of 0.1, 0.3 and 0.4, which could effectively protect C/C composites from oxidation at 1773 K in air for more than 100 h. The mass loss of the coated samples was primarily due to the reaction between C/C matrix and oxygen diffusing through the penetration cracks in the coating.