Helium ion irradiation at 200 °C, 350 °C and 550 °C was performed on a reduced activation ferritic/martensitic (RAFM) steel to investigate the evolution of voids during irradiation. Experimental results showed that radiation damage had a bell-shape distribution along the depth. Both void size and density increased as vacancy production rate increased. When irradiation temperature rose, void size increased and void density decreased. Faceted voids were observed at 550 °C. Voids aggregation at grain boundaries (GBs) and void denuded zones (VDZs) were observed at high temperatures. A phase field model was employed to investigate the void evolution mechanism. The simulation results showed that the void shape may be attributed anisotropic void interface energy. During irradiation, as time goes on, void evolution can be divided into three stages: incubation stage, nucleation stage and growth stage.