The mutual aid group, as supported by the social worker, emerges to play a vital role in helping group members reduce their deviance or behavioral problem. However, how the collaboration of the group and social worker accomplishes the reduction has remained uncharted. Based on social capital theory, mutual aid and cohesion within the group and social workers’ specific aid for the group are likely responsible for the reduction. The test of such hypotheses relies on a two-wave panel survey of the members of 60 mutual aid groups who had deviant behavioral problems, located in Hong Kong, China. These groups had 241 youths completing both initial and 1-year follow-up surveys. Results manifested the direct or unconditional contributions of mutual aid, group cohesion, and social workers’ specific aid to reducing deviance. Hence, social workers can enhance the effectiveness of the mutual aid group in reducing youths’ deviance.