Purpose: Body Mass Index (BMI) is the most convenient tool to assess body composition, and the Ministry of Education adopts it to evaluate body composition of elementary students. Due to the increasing rate of obesity prevalence, although height and weight is periodically measured, it is recommended to further monitor whether the development of BMI in elementary students is normal to determine the necessity of modification or treatment as early as possible. Methods: The present article reviewed related articles of height and weight, investigated the development of BMI with growing in elementary students, and explored the prevalence of overweight and obesity. Summary: The 85(superscript th) percentile of BMI in 10-13 yr boys announced by the Ministry of Education was 23-25kg/m^2, which corresponded to the 95(superscript th) percentile in WHO's report (22.9-24.8kg/m^2). The data from girls' side was quite similar. According to this information, the data from Ministry of Education might underestimate the situation of overweight and obesity in Taiwan elementary students. Hence, the opportunity of prevention and treatment for overweight and obesity early was missed. The development of elementary students was influenced by diverse factors, such as environment, nutrition, physical activity, inheritance, and so on. Because childhood obesity is harmful to health, we should encourage elementary students to engage in active living, healthy diet, and sufficient physical activity.