Objective To compare the changes in body composition, including fat mass index (FMI) and fat free mass index (FFMI) during puberty development of obese and normal-weight children in China, and to explore the effect of age and gender on body composition. Methods A total of 356 children at the age of 7-15 years were enrolled in this study. Body composition of 10 normal-weight and obese children in each age group was measured by dual-energy x-ray absorptiometry (DEXA). FFMI and FMI were calculated according to the following formula: FFMI (kg·m-2)= FFM(kg) / height2 (m2) and FMI (kg·m-2)= FM (kg) / height2 (m2). Results The fat mass and fat free mass of obese children were significantly higher than those of normal-weight children (P<0.05). The FMI and FFMI of obese children increased significantly with age and were higher than those of the same sex, gender, and age normal-weight children (P<0.05). Conclusion The levels of fat mass, fat free mass, FMI, and FFMI are different in obese and normal-weight children, and gender effects are significant in boys having higher levels of these indicators than in girls. FFMI and FMI can be used as monitoring indexes in weight control of obese children. Objective To compare the changes in body composition, including fat mass index (FMI) and fat free mass index (FFMI) during puberty development of obese and normal-weight children in China, and to explore the effect of age and gender on body composition. Methods A total of 356 children at the age of 7-15 years were enrolled in this study. Body composition of 10 normal-weight and obese children in each age group was measured by dual-energy x-ray absorptiometry (DEXA). FFMI and FMI were calculated according to the following formula: FFMI (kg · m-2) = FFM (kg) / height2 (m2) and FMI (kg · m-2) = FM (kg) / height2 mass and fat free mass of obese children were significantly higher than those of normal-weight children (P <0.05). The FMI and FFMI of obese children increased significantly with age and were higher than those of the same sex, gender, and age normal -weight children (P <0.05). Conclusion The levels of fat mass, fat free mass, FMI, and FFMI are different in obese and normal-weight children, and gender effects are significant in boys with higher levels of these indicators than in girls. FFMI and FMI can be used as monitoring indexes in weight control of obese children.