Thermal diffusivity of two kinds of two-directional carbon-carbon composites were studied after neutron irradiation at temperatures 640-1070°C to a maximum neutron fluence 1.98 10 2 5 n/m 2 (E > 29 fJ, 0.18 MeV) in the JMTR (Japan Materials Testing Reactor), The decrease of thermal diffusivity was observed after irradiation, and its significant decrease occurred at low neutron fluence, followed by gradual decrease with neutron fluence. The changes in thermal diffusivity, which were normalized to unirradiated values, were large at low measurement temperatures and decreased with temperature and also increased with neutron fluence. The changes in thermal conductivity related to thermal diffusivity are also discussed.