In this study, vapor of essential oil obtained by the hydrodistillation of Hypericum perforatum L., 1753 (Malpighiales: Hypericaceae) was tested on the different stages of Tenebrio molitor L., 1758 (Coleoptera: Tenebrionidae). The larvae, pupae and adult stages of T. molitor were exposed to different doses of H. perforatum essential oil for 24 h. After exposure, mortality rate, LC50, LC90 and LC99 values, antioxidant enzyme activities [superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferase (GST) and glutathione peroxidase (GPx)], acetylcholinesterase (AChE) activity and malondialdehyde (MDA) levels were measured in the insects. Tenebrio molitor was cultured at Gazi University Department of Biology and all analyses were done in Yozgat Bozok University in 2017 and 2018. The results indicated that the pupae of T. molitor were the most tolerant and adults were the most sensitive. Mortality increased with the increasing concentration of essential oil. Also, increasing doses of essential oil caused decreasing in SOD, CAT, GST GPx and AChE activities and increasing in MDA level. These results indicate that essential oil of H. perforatum can be used against T. molitor in a pest control program.