To examine the effects of oyster extract (OE) on ATP production in a human hepatocytes, Hep-G2 cell was cultured in Dulbecco’s modified Eagle’s basal medium or the basal medium containg OE. The added OE enhanced ATP production in the human hepatocytes. To examine the effect of OE on ATP production in mouse skeletal muscle and activity of mitochondria, female 9-week-old C57BL/6 N mice were divided into two groups and fed AIN-93G basal diet or the basal diet containing 2% (w/w) OE for 7 days. Muscle of the OE-fed mice had significantly higher ATP production and citrate synthase activity than non-OE-fed mice. These results indicate that the OE enhanced the mitochondoria activity and production of ATP in the muscle and led a possibility that OE has fatigue-relieving properties.