The purpose of this study is to examine the mediating effects of the relationship between early adolescents and their parents; the relationship between early adolescents and their teachers; and gender differences on the association between self-esteem and cooperation of middle school students. Structural relationships among self-esteem, parental relationship, teacher relationship, and cooperation were revealed using 2,590 first-year middle school students’ (1,405 male students, 1,185 female students) responses from the Korean Children and Youth Panel Survey, 2018. Multi-group analysis was used to analyze gender differences. Results showed self-esteem’s direct effect on cooperation and the mediating effects of parental relationship and teacher relationship between self-esteem and cooperation. Self-esteem not only has a direct effect on the cooperation of early adolescents, but also has an indirect effect through parental relationship and teacher relationship. Furthermore, it was confirmed that the mediating effect of teacher relationship was greater than parental relationship. Lastly, gender differences were identified only in the effect of self-esteem on cooperation. Based on these results, implications and suggestions were made regarding the mediating effects of parental relationship,teacher relationship, and gender differences in the effects of early adolescents self-esteem on cooperation.