[en] Material selection, selection of suitable structure and design method for high temperature structure are very important items for the design of heat-resistant structure for HTGRs. Structural metallic materials should be selected by considering manufacturing achievement and application experience, nuclear compatibility and endurance against specified environment. For these purposes, stabilized high temperature alloys, such as Inconel-625, Incolloy-800 and its family alloys, are expected as candidate. These alloys and others are under testing regarding the compatibility with coolant helium, the structural behavior in high temperature helium environment and to radiation dose. Structural design is mainly focused on the following points; in case of the components for high temperature service, heat-resistant and pressure-resistant structures should be separated, suitable heat insulators should be placed effectively, concentric double piping structure may be applicable to meet both requirement cited above, and the concentration of heat-generating core and heat-utilizing equipments should be carried out as far as possible. The design method for high temperature structure is in progress. Material behavior or creep and other performances at high temperature and in helium environment have been investigated, but the data on the materials corresponding to each fracture mechanism should be accumulated, such as the coupled effect of creep and fatigue, creep buckling, and so on. The more reliable design criteria and method will be completed in near future with these research and development works. (Iwase, T.)