In a very high temperature gas-cooled reactor developed at the Japan Atomic Energy Research Institute, reactor components, such as heat transfer tubes (Hastelloy-XR) of an intermediate heat exchanger, hot duct liners (Hastelloy-XR), core support plates ( Cr-1 Mo steel), control rod sheaths (Hastelloy-XR), orifice devices (SUS 304), fuel blocks (graphite), and others, are exposed to helium gas coolant with a temperature of 1000°C and a pressure of 4.1 MPa. The relative sliding movements of the structure, which are stimulated by flow-induced vibration, constraint force, and thermal expansion, might cause unfavorable friction and wear. Sliding wear tests were carried out on PGX graphite, Cr-1 Mo steel, and heat- and corrosion-resistant Hastelloy-XR in 500 to 1000°C. Environmental helium gas pressures of 0.2 and 4.1 MPa were chosen to compare the influence of the pressures. The effects of four different impurity gases (O2, H2, H2O, and CH4) on tribological behavior were studied, each gas concentration being varied up to ∼103 ppm. The specimen was a hemisphere-on-plate type, the plate being oscillated with a 5-Hz frequency and a 0.5-mm amplitude under a 9.8-N contacting load. The test duration was 3 h. In the case of Hastelloy-XR against itself, wear was adhesive in general, but the friction coefficient decreased to ∼0.3 in the environment with high-O2 concentration, and a relatively thick oxide film was found on the sliding surface. The results of calorized Hastelloy-XR against PGX graphite showed little dependence on impurity gas, and a lower value friction coefficient of ∼0.1 was obtained. In Cr-1 Mo steel against PGX graphite, thin layers of Fe2O3 and/or Fe3O4 were formed on the metal surfaces in the environment containing O2, and the friction coefficient gradually increased with high-O2 concentration. The case of PGX graphite against itself gave a low friction coefficient of ∼0.1 in the environment of high-O2 concentration, whereas in other impurity gases the value was ∼0.4.