Physical constitution consists of biologically different components, such as fat mass (FM) associated with various risks and fat-free mass (FFM) associated with physical fitness. Given a hypothesis that body composition to provide the best physical fitness is the healthiest, we estimated the relationship between fat-to-lean ratio (FLR= FM/FFM) and physical fitness outcomes in school children. Five hundred and three 4^th grade school children aged at 10 years were recruited and their body composition were measured by bioelectrical impedance analysis. They underwent physical fitness test including grip strength, sit-up, sit & reach, side step, 20m shuttle run, 50m dash, standing broad jump, softball throw. According to the sexes, the subjects were classified into 5 ranks (quintile) based on their heights, and subsequently stratified into 4 groups (quartile) in each height-rank (quintile) groups based on their FLR. By grouping the same FLR quartile group in 5 height-ranks (quintile groups), quartile groups (FLR-1 to FLR-4) were compared among their outcome of physical fitness test. Without relation to heights and FFM, 20m shuttle run as an index of cardiorespiratory fitness, 50m dash and standing broad jump showed significant difference among groups in stepwise manner in both boys and girls, where the most lean group had highest capacity. FM and FFM have to be discriminated each other, because of their characteristic biological significance. FLR, strongly correlated to aerobic power, is a useful index to estimate body composition associated with physical fitness.