The low insecticidal activity of rotenone against Spodoptera litura has been studied while its mechanism was not.To demonstrate the biochemical mechanism of the high tolerance of S.litura to rotenone and determining an effective synergist to improve the insecticidal activity of rotenone, this study was carried out.Result shows the cotoxicity coefficient of rotenone + cinnamon oil was 213.52.After rotenone treatment, a considerable amount of rotenone was discharged, but extremely low levels were found in the ventral nerve cord and brain.By contrast,the rotenone + cinnamon oil treatment group showed an increased amount of rotenone in the ventral nerve cord and brain.The midgut cells of rotenone and control groups exhibited no significant differences, whereas those of the rotenone + cinnamon oil group were extremely different because of the larger spaces between them.These findings suggest that rotenone alone could not effectively penetrate the midgut, which may be attributed to its low toxicity against S.litura.The rotenone + cinnamon oil mixture could change the midgut cell spacing and their membrane, thus effectively increasing rotenone toxicity.