Abstract: It has been reported that, with the prolonged photophase in the photoperiod experienced, the content of hydrogen peroxide (H₂O₂) in the prepupae of Cotesia vestalis (Haliday) increased, while the diapause incidence in their progeny decreased. In order to further explore the function of H₂O₂ in the diapause induction process of C. vestalis, we treated the prepupae obtained under a short photoperiod of 8L:16D with different concentration of H₂O₂ and measured the content of hydrogen peroxide and activity of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), as well as the diapause incidence in their progeny. The results showed that when the prepupae were treated with exogenous H₂O₂, the diapause incidence of their progeny declined when they were reared under a short photoperiod at 13℃. The diapause incidence in the progeny generation decreased from 83.76% in the control group to 35.22% in the group treated with 1.6 mol/L exogenous H₂O₂, close to the diapause incidence (12.24%) of their progeny whose maternal generation were reared under a long photoperiod. The content of H₂O₂ in the prepupae increased with the increasing concentration of applied exogenous H₂O₂, similar to the effect of prolonged rearing photoperiod; while the activities of SOD decreased and those of CAT in the prepupae increased with the increasing concentration of exogenous H₂O₂, just opposite to the effects with increased rearing photoperiod. The POD activity showed no significant difference between the prepupae treated at a low exogenous H₂O₂ concentration (0.2 mol/L) and the control. However, when the prepupae were treated at a high concentration (1.6 mol/L), the POD activity was the same as or even lower than the control at 4 h post-treatment, but was much higher at 20 h after treatment. The results indicated that exogenous H₂O₂ treatment may play a similar role as the long photoperiod in the process of diapause induction in C. vestalis, by preventing diapause occurrence, but has a different way in the effects on the three antioxidant enzymes.