In order to investigate the existing state of vacancies in cascades Fe and Ni specimens were irradiated with neutrons at low temperature (20K) and the isochronal annealing experiment was made at low temperature (100K) by positron annihilation lifetime measurement. Positron lifetime at low temperature region was found to correspond to a single vacancy both in Fe and Ni by comparing with the results of the electron irradiation experiment. In specimens irradiated with neutrons vacancy clustering started at lower temperature than that in the electron irradiation, which shows that vacancies in cascades are isolated from each other, but of high density. The neutron irradiation experiment made for Fe-0.22%Cu specimen showed that Cu atoms of this concentration does not affect the clustering behavior of vacancies, which suggests that vacancy concentration is much higher than Cu concentration of 0.22%.