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A SiC whisker-toughened MoSi_2-SiC-Si coating was prepared on carbon/carbon (C/C) composites surface by a two-step technique of slurry and pack cementation, and the effects of thermal shock and oxidation on the mechanical property of the coated C/C were studied. The flexural strength of C/C composites was improved by 6.8% after coated by SiC whisker-toughened MoSi_2-SiC-Si. After thermal cycle between 1773 K and room temperature in air for 10 times, the mass loss of the coated sample was 5.08% and the percentage of remaining strength was 81.97%. After oxidation at 1773K in air for 60 min, the mass loss of the coated sample was 2.57% and the percentage of remaining strength was 89.63%. The decrease of the flexural strength during the thermal cycle and oxidation tests was primarily due to the oxidation of C/C substrate resulting from the cracking of coating.