Effects of damage rate on the formation of voids and interstitial type dislocation loops in pure metals, such as Au, Cu, Ni, Fe and V, and alloys of Ni and V, were investigated. The specimens were irradiated at the Kyoto University Reactor (KUR) with improved temperature control. The dose rates of neutron irradiation were 4.4×1015n/m2s (E > 0.1 MeV) and 3.9×1017 n/m2s (E > 0.1 MeV) at 573 K. By the total dose of about 7×1021 n/m2 (E > 0.1 MeV), the loop density increased with increasing dose rate, whereas the growth behavior of microvoids depended on the dose rate and irradiated materials. In Cu, Fe and V, the concentration and the size of microvoids increased with increasing dose rate. In Au, the difference of microvoid formation between two dose rates was small. While in Ni, Ni-Cu and Ni-Ge, the size of microvoids increased with decreasing dose rate. The behaviors of formation of microvoids and dislocation loops were analyzed by the rate theory. It was clarified that the microvoid formation strongly relates to the mobility of vacancies. The calculation also showed that the nucleation of dislocation loops increases with increasing dose rate.