F82H low-activation martensitic steel was irradiated in the Swiss Spallation Neutron Source (SINQ) Target-III to irradiation doses of 10-12 dpa in a temperature range of 140-360 degreesC. Transmission electron microscope observations have been performed to study the radiation effects on the microstructure at different irradiation temperatures. The results show: (1) High-density helium bubbles of size greater than or equal to 1 nm are observed in the samples irradiated at temperatures greater than or equal to 175 degreesC. The size of bubbles increases with increasing irradiation temperature, but the density remains almost constant. (2) The irradiation temperature is the controlling parameter for the amorphization of the precipitates in F82H steel. Present work indicates that the amorphous temperature for typical precipitates (M23C6) is about 235 degreesC corresponding to about 0.3T(m) (T-m is the absolute melting point temperature) of the steel. (3) With increasing irradiation temperature, the mean size of the defect clusters remains almost constant below 235 degreesC but increases rapidly at higher temperatures. The density of defect clusters shows no significant change up to 255 degreesC and then decreases rapidly above this temperature. (C) 2002 Elsevier Science B.V. All rights reserved.