Creep and stress rupture properties are determined for the candidate materials to be used in high-temperature gas-cooled reactor (HTGR) components. The materials and test methods are briefly described based on experimental results of test durations of ∼20 000 h. The medium creep strengths of the alloys lnconel-617, Hastelloy-X, Nimonic-86, Hastelloy-S, Manaurite-36X, IN-519, and Incoloy-800H are compared showing that lnconel-617 has the best creep rupture properties in the temperature range above 800 °C. The rupture time of welded joints is in the lower range of the scatterband of the parent metal. The properties determined in different simulated HTGR atmospheres are within the scatterband of the properties obtained in air. Extrapolation methods are discussed and a modified minimum commitment method is favored.