The purpose of our study was to investigate whether the Trp64Arg polymorphism in beta3-AR gene and the -3826A/G polymorphism in the UCP1 gene were associated with the reduction in energy expenditure and fat oxidation both in resting and aerobic exercise in Japanese. Eighty-six nonobese young healthy Japanese were recruited. Energy expenditure was measured using indirect calorimetry. The subjects performed an aerobic exercise program at 60% of their maximal heart rate for 30 minutes. The level of fat oxidation at rest and aerobic exercise of the male subjects with Trp/Arg of the beta3-AR gene was significantly lower than that of the Trp/Trp genotype. No difference in FO(0-30) was observed in the female subjects. There was no association between UCP-1 polymorphism and energy expenditure during aerobic exercise. It was revealed that the Trp64Arg polymorphism in beta3-AR gene is associated with reduction of fat oxidation both in resting and aerobic exercise in healthy, young Japanese males.