Despite numerous studies regarding overweight or obese children, only a limited number of studies have investigated the effect of underweight. The purpose of this study is to investigate the determinants of underweight among school-aged children. A total of 493 students (86 underweight and 407 normal weight students) aged 11 to 13 years were included in our study. Socio-demographic characteristics, eating habits, health information, self-perception of weight, weight-control efforts and birth-related information were collected by using survey questionnaires for children and parents. Dietary information was obtained by two 24-hour food records, which were completed by both children and their parents. The prevalence of underweight was significantly higher in girls than boys, and the frequency of medical treatment and flu symptoms were higher in underweight children than normal ones. Overall, girls tended to overestimate their own weight; this misclassification was greater among underweight girls. Birthweight was positively correlated with current weight (P<0.05) and height (P<0.01) in girls, but these correlations were not seen in boys.In conclusion, underweight girls had inappropriate self-perception of weight, and underweight in girls may be related with birthweight and inadequate dietary intakes. Therefore, it is important to build a well-designed framework that integrates efforts of home, school, and community to maintain a healthy weight with balanced diet and exercise throughout the lifetime.