Aphids are one of the major pests in agricultural crops. A number of synthetic pesticides have been used for control of aphids in agriculture, but increasing public concerns over their adverse effects on the environment have required more environmentally-friendly methods for pest management. In this study, we examined plant oil formulations for the control of green peach aphid (Myzus persicae). Oil formulations were prepared by hydrolyzing the plant oils in ethanolic KOH solution and diluted at the rate of 1:500 for aphid control. The oil formulations showed aphid mortalities ranging from 24.44 to 43.33% in vitro. Significantly increased aphid mortalities were observed by the treatment of oil formulations combined with low-dosed imidacloprid. No significant difference in the aphid mortality was observed between the oil formulations. Mass spectrometry analyses of aphids treated with the low dosedimidacloprid plus the plant oil formulations detected similar concentrations of imidacloprid between the treatments. In field trial bioassays against aphids, significantly decreased aphid population were observed in the pepper plants treated with soybean oil formulation combined with the low-dosed imidacloprid, while aphid population dramatically increased in the pepper plants treated with the low-dosed imidacloprid alone. These results suggested that the plant oil formulations can be used as an environmentally-friendly method for enhancing the insecticidal effectiveness, which may play a role in reducing the use of synthetic pesticide in agriculture.