The toxicological activity of four plant oils, Matricaria chamomilla, Origanum majorana, Carum petroselinum and Salvia officinalis were evaluated against all larval instars and pupae of Aedes caspius and Culex pipiens. The plant oils were tested at different concentrations of 100, 200, 400, 800 and 1600 ppm at different time intervals (24, 48 and 72hrs). The obtained results showed that, all tested plant oils had prominent mosquitocidal activity against A. caspius and C. pipiens. Mortality increased by increasing concentration and time of exposure. M. chamomilla showed highest larvicidal activity after 24 hrs. for all larval instars of A. caspius and C. pipiens. Higher mortality was observed in 1st instar larvae than all other immature life stages with all oils. The developmental periods were extremely prolonged after treatment with the lowest concentration of all used oils. Adult emergence was completely stopped at 800 ppm of M. chamomilla and C. petroselinum against A. caspius. It was concluded that, the used oils have toxic effects against larvae and pupae of A. caspius and C. Pipiens, altered developmental periods, pupal rate and adult emergence with superiority of M. chamomilla and C. petroselinum. It was recommended that, M. chamomilla and C. petroselinum have the potential to be used for larval and pupal control of A. caspius and C. pipiens.