The reactor pressure vessel (RPV) of the HTTR is 5.5m (inside diameter), 13.2m (inside height), and 122mm (shell thickness). The RPV contains core components, reactor internals, reactivity control system, etc.2 1/4Cr–1Mo steel is chosen as the material for RPV. The temperature reaches about 400°C at normal operation. The fluence of the RPV is estimated to be less than 1 × 10 17 n/cm 2 (E > 1MeV) and so irradiation embrittlement is negligible, but temper embrittlement is not negligible. For the purpose of reducing embrittlement, content of some elements must be limited in the 2 1/4Cr–1Mo steel for the RPV; embrittlement parameters, J-factor and X¯ are used.In this paper, design and structure of the RPV are reviewed first. Fabrication procedure of the RPV and its special feature are described. Material data on the 2 1/4Cr–1Mo steel manufactured for the RPV, especially the embrittlement parameters, J-factor and X¯, and nil-ductility transition temperatures, T NDT , by drop weight tests, are shown. In-service inspection and results of R&Ds are also described.