Abstract: The tritium permeability of CLAM steel, one of the low activated ferrite/martensite steels (RAFM steels) made in China, have been experimentally measured by means of the gas evolution permeation technique over the temperature range of 573-823 K. The resultant permeability F_T is 2.57×10^-8exp(-38639/RT), the deduced diffusivity D_T is 1.17×10^-7 exp(-22011/RT) and Sieverts’ constant S_T is 2.2×10^-1exp(-38639/RT). Again, there is an obvious isotope effect on D-T mixture permeation in which the permeability of deuterium is larger than that of tritium. The permeation separation coefficient α_DT is 1.42 for D-T, and α_HT is 3.76 for H-T.