In this study, epigallocatechin-3-gallate (EGCG) was examined for the first time for its anti-aging effect on middle-aged male Fischer 344 rats as a dietary supplement at 50 (low dose) and 500 (high dose) mg/kg/day over a 6-month period. Such levels of EGCG concentration were well-tolerated by rats without causing tissue damage or dysfunction in the liver and kidney, as evaluated by histopathological and biochemical observations. Compared to the rats in the low-dose and control groups, rats fed with high-dose EGCG showed a significant decline in the concentration of 8-hydroxy-2'-deoxyguanosine in the plasma while maintaining a better mitochondrial potential in the peripheral lymphocytes and preventing the deletion of ND4 region from mitochondrial DNA in the liver. The protective effects of high-dose EGCG against oxidative stress were comparable with the effects of caloric restriction, a well-established dietary intervention that retards aging. However, the supplementation of EGCG influenced merely the antioxidative enzyme activities and their gene expressions in rats, suggesting that EGCG may either function as an antioxidant itself or regulate other bioprocesses, including energy metabolism, biosynthesis, and stress response, as shown in the gene profiling analysis of microarray data. Thus, the present study provides preliminary information on the anti-aging property of EGCG in male Fischer 344 rats.