Abstract: The reduced activation ferritic/martensitic (RAFM) steel is a promising structure material candidate for tritium blanket module in fusion reactor. Hydrogen isotopes transport within the material must be well characterized for affecting the tritium recovery from the module, the self-sufficiency of the fusion fuel recycling and the radiological safety. The deuterium transport parameters of diffusivity, Sieverts constant and permeability in CLAM (China low activation martensitic) steel which is one of RAFM steel developed in China were experimentally measured by means of the gas permeation technique over the temperature range of 573-873 K with driving pressure from 0.03 to 0.1 MPa. The resultant transport parameters are: A Sieverts constant of S(mol/(m³•Pa^0.5))＝0.264exp(-22 447/RT), a diffusivity of D (m²/s)＝1.38×10^-7•exp(-17 271/RT) and a permeability of Φ (mol/(m•s•Pa^0.5))＝3.64×10^-8•exp(-39 718/RT). There is an obvious isotope effect on H-D mixture permeation in which permeability of deuterium is less than that of protium. Experiments and theoretical analysis reveal that thermal release at high temperature shall be an efficient way for removal of the deuterium dissolved in the steel.