The relationships between body composition and selected physical fitness and motor ability elements were determined in junior high school boys and girls. The subjects were five hundred and twenty nine boys and girls aged 12∼15 years. 12 physical fitness and motor ability elements were tested and skinfold thickness was measured at five sites using an Langer caliper. Impedance(Z) was also measured using a tetrapolar bioelectrial impedance plethysmograph(Gilwoo, GIF 881). Body density was calculated from the formula of Watanabe et al(1993). Boys had significantly less fat and performed significantly better than girls on each of the performance tests. For the group of boys and girls, moderately strong relationships between %fat and performances on each of the tests were found, with correlations ranging from 0.144 to 0.640. Variables which represented muscular power and endurance were negatively affected by %fat, while muscular strength was positively related to %fat. Flexibility variables were found not to be affected by %fat. This is also confirmed in obese group. In order to further examine the effects of %fat on physical fitness and motor ability, the subjects were categorized into three groups according to %fat : lean(n=22), standard(n=310), and obese(n=192). The result of comparison among groups clearly indicated that the obese group was significantly inferior in muscular power and endurance but was better in muscular strength than the other groups. The relative amount of body fat. was found to be one important factor that affects many physical fitness and motor ability elements.