The aim of this study was to evaluate the body composition difference and obesity prevalence in grade 5 to 8 by gender. A total of 1,333 students, 707 boys and 631 girls, were recruited from one elementary and one middle school in Seoul, Korea. Anthropometric measurements including height, weight, lean body mass, and body fat were assessed. Height and weight were increased by grade, but Body Mass Index (BMI) by grade showed no significant difference among boys. Lean body mass had growth spurt between 7 and 8 grade among boys compared to gradual increase among girls. Overweight and obesity prevalence were higher among elementary schoolers than middle schoolers. Overweight (BMI for age> $85^{th}$ percentile) prevalence was about $44\%$ among $5^{th}$ grade boys compared $29\%$ among same grade girls. These results provided that there were prominent difference regarding body growth and obesity prevalence in gender and grade. Further studies for children and adolescents should be considered their body composition change.