Results of experiments in laboratory showed that margosan O possessed strong insecticidal properties and growth regulatory effects on the 3rd instar larvae of the diamondback moth (DBM). The mortality of the larvae and pupae was 84.21%when the 3rd instar larvae of DBM were fed by the leaves of vegetable sprayed with 15 mg/L margosan O. The pupation, emergence and fecundity of DBM were seriously supppressed by margosan O. Experimental results also proved that the LC 50 was 36.90 mg/L and 28.77 mg/L respectively, on resistant strain and susceptible strain of the 3rd instar larvae of DBM.