Obesity in the adolescence has a negative effect on ego-identity and self-esteem and thus side effects caused by obesity may negatively affect scholastic achievement in middle and high schools. The study of obesity is important in that low educational achievement in the adolescence may be a cause of low human capital accumulation and thus low wage in the labor market. The purpose of this paper is to empirically analyze how obesity is related with educational achievement for middle and high school students. We employ student level data from KEEP(Korean Education and Employment Panel) surveyed in 2004. The results of study are summarized as follows. Obesity significantly has a negative effect on academic performance for senior students in middle and high schools. This negative effect is estimated to be more serious for middle school students and for female students. Controlling other factors, average percentage grade is 8.69% lower for students in obese group and particularly the grade is on average 14% lower for obese female students in middle school. Since empirical results could reflect the economic harm of obesity, government need to make efforts to prevent excessive obesity of students in the adolescence.