Silk peptides (SP) have been reported to decrease body weight and accumulate fat. We investigated the effects of SP administration by using an open circuit calorimetry system on resting energy expenditure and substrate utilization in resting mice for the duration of 24 h. Seven-week-old male ICR-mice were orally administered SP (800 mg/kg) for 2 wk and were subjected to endurance training. The results indicated that not only was oxygen uptake higher in the SP group than in the CON group (*p<0.05), but also the respiratory exchange rate was lower than that in the CON group for the duration of 24 h (**p<0.01). Moreover, fat oxidation was increased in the SP group. Body weight of the SP group was significantly decreased compared to that of the CON group (*p<0.05). These results suggest that intake of silk peptides increases fat oxidation during rest in exercised mice. Intake of silk peptides is considered to be a favorable supplement for athletes in training. In particular, it would be an effective supplement for athletes who require weight loss along with an increase in muscle mass.