This study is to offer a criterion of obesity judgement consisting of three groups with percentage of BMI(Body Mass Index) of children and sugges the validity of BMI through analyzing correlation of the elements of physical strength and BMI. For that end, we contacted 2048 male and remale 5th and 6th graders from 20 elementary schools located in Kanwondo and Kyungfido and sub-categorized them into obesticy warning group with BMI 85% to 94% obesity group with BMI 95% or more, and normal group with BMI 85% or less. Through compatin each group\'s record of 50m run sit-up , standing broad jump, trunk flexion and long - last run analyzing correlation of the elements of physical strength and BMI , we concluded as mentioned in the following. 1) In the case of 5th grade female students, those of 21.23kg/m2 of more belong to obestiy warning group and 24.51kg/m2 of more obesity group, while make students of 22.8kg/,m2 of more are categorized into obesity warning group and 25.65kg/m2 of more obesity group. 2) In the case of 6th grade female students, those of 23.04 kg/m2 or more belong to obesity warning group and 25.65kg/m2 or more obesity group , while male students of 21.88kg/m2 or more are categorized into besity warning group and 24.28kg/m2 or more obesity group. 3) It is found that there is a correlation between all the elements of physical strength andBMI . and in particular long-last run has a higher correlation than other elementts. 4) All the elements except trunkg flexion are found to vary from groups. From the finkings of the study , vailidity of BMI for obesity judgement is proved so it is useful to use BMI for obesity judgement of children at school.However , it is more reasonable to use an absolute BMI criterion according to the level of physical strength in comparison with relative obesity judgement base on percentage in the case of children.