Abstract: Glutathione peroxidase (GSH-PX), which was found in many animal tissues, was regarded as a major protective system against lipid peroxidation induced endogenously and exogenously. The result indicated that GSH-PX activity in both Helicoverpa armigera and Ostrinia furnacalis could be induced and activated by α-T, a typical phototoxic thiophene derivative from species in the Asteraceae, whenever exposure to ultraviolet light or not. It suggested that GSH-PX plays some important roles in providing protection to insect against harmful oxygen derivatives. The potential role of GSH-PX against the oxidative stress caused by α-T in the mediation of singlet oxygen was discussed.