Abstract: To research the resistance of China low activation martensitic (CLAM) steel to irradiation swelling, the microstructure of CLAM steel irradiated by high energy elections at 450℃ was in-situ observed on a high voltage electron microscope. The results indicate that a lot of voids and dislocation loops form by irradiation. The dependence of the amount and size of these damages on the irradiation dose are evaluated and discussed. The irradiation swelling is about 0.26% and thus CLAM steel is considered as a material with high resistance to swelling.