Abstract: High displacement damage is produced in structural materials when irradiated with high energy neutron. And the property degrading is induced in materials because of lots of irradiation defaults. In order to investigate the irradiation defaults, high energy electron irradiation experiment on China low activation martensitic (CLAM) steel was carried out with high voltage electron microscope (JEM-ARM1300) to simulate the high displacement damage and to in-situ observe the change of microstructure with irradiation dose increasing. Dislocation loops were observed during the irradiation process. Changes of size and density of dislocation loops with the irradiation dose increasing were analyzed.