Abstract In a blanket made of SiC/SiC composite, helium gas employed as coolant may leak into plasma, resulting in fuel dilution. The helium gas permeability of SiC/SiC composite has to be measured to qualify SiC/SiC composite as a blanket material. Since the SiC/SiC blanket receives heat cycles due to plasma start up and shut down phases, the change of helium gas permeability after the heat load has to be investigated. Heat cycles with different highest temperatures and heating rates were applied to SiC/SiC composite with very low permeability, and the change of permeability was measured. No increase in the permeability was observed for an operation temperature of SiC/SiC blanket at 1100 K. The increase of permeability was observed only when the maximum temperature was very high, 1300 K, and when the heating rate was also large, 10 K/s. The operational regime without increase of permeability was determined based upon the present data.