[en] The JMTR (Japan Materials Testing Reactor) is a 50 MW tank type reactor, cooled and moderated by light water. It started the irradiation service in 1970. The JMTR has been providing high neutron flux in core positions for irradiation testing of reactor materials and fuels, and for radioisotopes production. In fiscal 1991, the JMTR operated of 5.1 cycles as planned. Extensive irradiation tests, including power ramping tests on LWR fuel rods were carried out. Furthermore, the development of new irradiation techniques and post irradiation techniques such as re-instrumentation of a thermocouple to an irradiated fuel rod and tube joint utilized the shape memory alloys were continued. The study for irradiation performance on fusion reactor blanket was started. For the JMTR core conversion to low-enriched uranium (LEU), the safety review was completed by the competent authorities, and the license was issued. The core of the JMTR will be converted to LEU fuels in 1993. (author)