Abstract: Reduced activation ferritic/martensitic (RAFM) steel is considered as the primary candidate of structural materials for the first wall of International Thermal Experimental Reactor (ITER) and fusion power reactors, many countries are developing different kinds of RAFM steels such as F82H and JLF-1 in Japan, 9Cr2WVTa in USA, EUROFER 97 in Europe and so on. Research on China low activation martensitic (CLAM) steel is also undergoing. The neutrons with energy of 14 MeV would produce a large number of crystal defects and in turn cause serious irradiation damages. At the same time, the neutron irradiation will also produce transmutation reactions of (n, p) and (n, α) and results in the formation of helium or hydrogen in materials. These gas atoms may promote the damage behavior during irradiation. This work used ion accelerator to implant ions of deuterium into CLAM steel at room temperature, followed by annealing at high temperature to make the defects develop into large dislocation loops. High voltage microscopy JEOL-1300 was used to investigate the microstructural change in deuterium ions implanted CLAM steel under 1 250 keV electron irradiation at temperatures from room temperature to 873 K. Under the electron irradiation, both of growth and shrinkage of the defect clusters produced by deuterium ions implantation were observed. The results show that both interstitial loops and vacancy loops are formed during the deuterium implantation because the first formed at lower annealing temperature is growing continuously under electron irradiation while the second formed at higher temperature is shrinking due to electron irradiation. The voids produced by the implantation were also studied.