A series of irradiation experiments to investigate the effects of neutron fluence and flux on reactor pressure vessel (RPV) steels has been performed using the Japan Material Testing Reactor (JMTR). The irradiation temperature has been precisely controlled at 290°C within an error of ±2°C during reactor operation. The neutron fluence and flux ranged from 9.3 × 1017 to 1.1 × 1020 n/cm2 and from 1.4 × 1011 to 6.3 × 1011 n/cm2/s, respectively. The material used in the present study was an A533B RPV steel in which the copper and phosphorous concentrations are 0.03 and 0 002 wt%, respectively. Charpy impact test results with one-third sized specimens showed no dose-rate dependence. Microstructural observation revealed that a higher irradiation temperature at around 340°C resulted in formation of rather large dislocation loops and smaller microvoids.