Measurement of body fat contents is essential for a judgement of obesity, which is defined as an abnormal increase of the fat content in the body. The fat content estimated from skinfold thickness was compared with the value calculated from the whole body potassium content which was obtained by the counting of whole-body radioactivity. The obese and non-obese women were chosen as subjects for this study. It became clear that the fat value estimated from the potassium content is higher than from the skinfold thickness. The relation between two calculated values were investigated by regression lines. It was obvious that the regression line of obese woman group vary with age, but not of nonobese group.