Social isolation can induce psychological behavior changes. It is interesting to know whether there is sex difference in responding to social isolation or not. The present study compared the behavior difference between male and female mice isolated for 1-4 months. The results showed that the isolated male mice had higher accounts of locomotor activity than the isolated female and group-housed ones. Both isolated male and female mice spent shorter time in the dark box than the group-housed mice in the light/dark test, and isolated male mice spent less time in the closed arms than isolated female and group-housed mice when isolated for 2, 3 and 4 months in the elevated plus-maze test. These results suggest that isolation induce an anxiolytic-like effect. The immobile time in the forced swimming test was shortened in male mice isolated for 1 and 2 months. Both isolated male and female mice showed shorter time in pentobarbital-induced loss of righting reflex and less body weight gain. These results demonstrated that there was a sex difference in psychological behavior changes in mice undergoing social isolation and the male mice were more easily affected by isolation.