This study examined the impact of academic achievement and maternal differential treatment on academic self-concept among adolescent siblings. The sample consisted of 438 students attending middle schools in Seoul (M = 15.2 yrs.), who had a sibling. Data were analyzed using Structural Equation Modeling. Academic achievement compared with a sibling and maternal differential treatment had direct effects on academic self-concept, indicating that children with higher academic achievement scores than their sibling and with perceived maternal differential treatment reported a significantly higher academic self-concept. Also, the relationship between academic achievement as compared with a sibling and academic self-concept was mediated by maternal differential treatment. These findings could be used in educational settings as a basis for improving the academic self-concept of early adolescents.