Abstract: CLF-1 steel is one of the most promising structure candidate materials in low activated ferrite/martensite steels(RAFM steels), and its hydrogen isotope permeation behavior is attracted much attention. In this work, hydrogen isotope permeation experiments were carried out on CLF-1 steel by gas driven hydrogen isotope permeation method. The effects of temperature, gas pressure and surface condition on the permeation behavior were investigated. The experimental results show that the diffusion process of hydrogen and deuterium in RAFM CLF-1 steel is controlled by bulk diffusion, and the relationship between permeability and temperature follows Arrhenius relationship; during the experimental test, the permeation flux of H₂ and D₂ decreases due to oxidation and defect trapping on the sample surface.