This paper describes the conceptual design and cost estimation of a 600MW(t) HTGR-GT power plant, which has been completed in the framework of the HTGR-GT feasibility study project in the duration of FY 1996 to FY 2000. The project is assigned to JAERI by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (former Science and Technology Agency) in Japan. The inlet and outlet gas temperatures in the reactor are 460°C and 850°C, respectively. Helium gas pressure is 6MPa. The gas turbine system type is an intercooled recuperative direct cycle. Designs of reactor and gas turbine are presented. The main feature of the plant is a relatively large 600 MW(t) HTGR, horizontal single shaft helium turbine and divided power conversion vessel, that is, a turbomachine vessel and heat exchanger one. Their main specifications and drawings are presented. As a result of cost estimation, an economically attractive construction cost and a power generation cost have been obtained.© 2002 ASME