Six plant oils of Mustard (Brassica campestris L.), Basil (Ocimum basilicum L.), Chamomile (Matricaria recutita L.), Anise (Pimpinella anisum L.), Orange (Citrus sinensis L.), and Rosemary (Rosmarinus officinalis L.) were evaluated in the laboratory for their toxicities against adults of the rust red flour beetle, Tribolium castaneum (Herbst) at the LC50 after 7 days post-treatment at 30±1° C. Toxicities of plant oils could be arranged in descending order as; Orange oil, Chamomile oil, Rosemary oil, Basil oil, Mustard oil and Anise oil was the least effective. The results of the effect of LC50 of Orange oil on blood cells of the larvae showed mild effect which appeared on cell membrane and the presence of empty cells. The LC50 of Orange oil on the total proteins, lipids and carbohydrates of the treated adults of T. castaneum showed significant decrease, and non-significant effect on enzymes activies (GOT), (GPT) and Acetylcholinesterase. A Significant increase in activity of Alpha esterases, Beta esterases, Trehalase and Amylase was recorded, while a significant decrease was recorded in the activity of Invertase enzyme.