Abstract: ObjectiveTo study the relatio nship between body composition and the development of sexual organ among boys in prepuberty.MethodsAll boys were divided into three groups(obese,median,lean)according on the percent of body fat that was calculated based on the skin fold.Finally,84 boys were filtered as objects and observed for four years.The measurement of skinfold thickness:Harpendenforceps.Radioimmunoassay(RIA)was used to determine testosterone.ResultsDuring the period of 4 years,the length of penis:lean group>median group>obese group,and there was significant difference of the length of penis between lean group,median group and obese group(P<0.05).There was significant difference of the width of penis between lean group,median group and obese group(P<0.05)from age 9.With the age increasing,the length of testicles increased,and there was significant difference of the length of testicles between lean group,median group and obese group.There was negative relationship between sexual organ and% BF.ConclusionObesity is associated with the maldevelopment of sex organs in prepuberty.