A TEM observation of fission neutron-irradiated copper at 300°C shows that the maximum size of stacking fault tetrahedra (sft) observed is 6 nm of edge length which corresponds to a cluster of 280 vacancies and the minimum size of voids is 2.2 nm in diameter which corresponds to a cluster of 470 vacancies. The result suggests that a vacancy cluster whose size is smaller than 300 vacancies grows to sft while a cluster whose size is larger than 500 vacancies relaxes to a void in 300°C-irradiated copper. A computer simulation of molecular dynamics (MD) with an isotropic EAM potential examined this model. It is found that a vacancy cluster that is smaller than 300 vacancy segregates to a (111) platelet and relaxes to an sft. Small vacancy clusters which are generated at damage cascade cores aggregate to spherically distributed vacancies for the size of more than 500 vacancies, and relax to several (111) platelets, which finally form a vacancy (111) polyhedron. Inside a polyhedral vacancy platelet, vacancies are confined and grow to a void at high temperature.