Abstract In the High-Temperature Engineering Test Reactor (HTTR), the hottest service temperature of class 1 metallic components exceeds 900°C under exposure to the primary helium coolant even at the normal operations, and control rods are also heated up to about 900°C at reactor scrams. For these components, Hastelloy X and Alloy 800H were selected from screening commercial heat-resistant superalloys. Hastelloy X is improved to have a superior compatibility with the primary coolant, and the improved version is called Hastelloy XR. The materials mentioned above and their service conditions are out of scope of the Japanese elevated-temperature structural design code, and further at the very high temperatures elastic-calculation-based design rules cannot satisfy the allowable limits due to very fast stress relaxations. Many research works on material characterizations and structural mechanics behavior, therefore, have been made in order to establish a high-temperature structural design code for both materials and also a creep analysis method for Hastelloy XR. Furthermore, a proposal was made against a key issue on tensile property. i.e., dynamic recrystallization for Hastelloy XR. This paper describes the results of research and development both on developing the metallic materials and on establishing the design code for these very high-temperature components.