In this study, we investigate the effect of dietary Royal Jelly (RJ) on tissue DNA oxidative damage and on the life span of C3H/HeJ mice. In C3H/HeJ mice that were fed a dietary supplement of RJ for 16 weeks, the levels of 8-hydroxy-2-deoxyguanosine (8-OHdG), a marker of oxidative stress, were significantly reduced in kidney DNA and serum. Secondly, we determined the effect of dietary RJ on the life span in C3H/HeJ mice. The 50% mice survivals of intermediate- (about 6 mg/kg weight) and high-dose groups (about 60 mg/kg weight) were reached at significantly longer times than that of the control group according to the generalized Wilcoxon test (p<0.05). The average survival times were 88 weeks for the control group vs. 79 weeks for the low-dose group (about 0.6 mg/kg weight), 112 weeks for the intermediate-dose group and 110 weeks for the high-dose group, respectively, showing that RJ extended the average survival time by about 25% compared to the control group. However, RJ did not extend the total life span. These results indicated that dietary RJ increased the average life span of C3H/HeJ mice, possibly through the mechanism of reduced oxidative damage.