Abstract: In terms of the radiological characterization survey in transition phase for the 101 Heavy Water Research Reactor (HWRR), the first research reactor of China, the radiological inventory of the structural components of reactor body was obtained by applying the calculation method combined with the sampling from the reactor structure material activated by neutron. The radiological inventory of the radio-contaminated surface of fluid system and reactor building was obtained by using radioactive measurement and surface sampling, scraping and smear. According to survey results, the information on the radiological inventory and the main radiological characterization of 101 HWRR were obtained.