Reduced activation martensitic steel specimens were irradiated by dual-beam from 0.1 to 50 dpa at 250 and 300 degreesC with 10 He appm/dpa. The in-situ microstructure observation shows that the bubbles were present after 1.7 dpa for 250 degreesC irradiation and after 0.1 dpa for 300 degreesC irradiation. The bubble size distribution is a Maxwell distribution, which evolves to a bimodal distribution with increasing damage dose. The transition occurs after 10 dpa at 300 degreesC irradiation and after 30 dpa at 250 degreesC irradiation. The bubble concentration and peak bubble size are similar for various doses at 250 degreesC, and are 3.8 x 10(21) m(-3) and 2.5 nm, respectively. The size of cavities at the second peak of the bimodal distribution is around 6 nm. The cavity size distribution increases and spreads out with damage dose increase. The dislocation loops are typical interstitial dislocation loops that evolve from black-white spots to visible interstitial loops. The size range of interstitial loops is between 2.5 and 11 nm and the dislocation loop density increases monotonically with the damage dose. (C) 2004 Elsevier B.V. All rights reserved.