The effects of cyclic temperature changes during neutron irradiation upon radiation induced microstructure evolution and resulting property changes of materials is very important from both fundamental and engineering viewpoints. Therefore, a technique that allows us to do the controlled temperature-cycle irradiation was developed in the Japan Materials Testing Reactor (JMTR). The technique meets the following requirements : (1) the temperature-cycle irradiation is to be performed under three different conditions by changing lower and upper temperatures; 200 --- 400℃, 300 --- 400℃ and 300 --- 450℃. (2) the number and period of the temperature-cycles are to be six for 24-day full irradiation and approximately 44 h/44 h at the lower/upper temperatures. (3) the temperatures of each specimen assembly are to be maintained at the lower temperatures before start-up of the reactor and at the upper temperatures during shut-down until the complete absence of reactor power. In this paper, the details of the irradiation rig, successful results and several problems to be overcome for future improvement are presented
