The purpose of this study was to examine the impact of peer support on conduct behavior and withdrawal and to investigate the mediating effect of negative automatic thought. The subjects included 461 students(boys 51%, girls 49%) from the 1st to 3rd grades of middle school in Seoul. Conduct behavior and withdrawal was measured by the Korean Youth Self Report and the Conners-Wells’-Adolescent Self Report Scales. Automatic thoughts were measured by the Korean Children Youth Automatic Thoughts Scale. Using Amos 20, structural equation modeling was conducted. The findings of the structural equation modeling was that peer support had an impact on conduct behavior and withdrawal. Peer support contributed to a reduction of negative automatic thoughts. Negative automatic thoughts had a mediating effect between peer support and withdrawal or conduct. We suggest that peer support affects conduct behavior and withdrawal through negative automatic thought. We also found gender differences; compared with girls, boys received less support from peers. They also had lower levels of negative automatic thoughts and withdrawal than girls. The further study should be considered to provide more correlation factors of peer support.