Abstract Single-crystal silicon was neutron-irradiated up to a fluence of 3.0 × 10 23 n/m 2 ( E n > 0.1 MeV) at 120–150 °C, and up to a fluence of 6.9 × 10 23 n/m 2 at 300 °C. Changes in macroscopic length and FT-IR spectra were observed after irradiation and after post-irradiation isochronal annealing up to 1000 °C. Irradiation-induced swelling was 0.01% in both specimens. Up to 1000 °C, relatively large shrinkage was observed around 600 °C in both specimens by precise dilatometric method. There was a difference of the recovery rate between two irradiation conditions. From the FT-IR spectra, it is supposed that the Si irradiated at 120–150 °C includes more small vacancy clusters than the Si irradiated at 300 °C, thus recovery of the former Si was faster than that of the latter Si.