Abstract: Together with the indoor toxicity test of lufenuron to the third instar larvae of Plutella xylostella (Lepidoptera: Yponomeutidae), the effect of lufenuron on survival, pupation and adult emergence of the tested insect was studied to fully understand its biological activities. The results showed that lufenuron had characteristics of high biological activity at low dosage, slow effect but lasting efficacy on P. xylostella. The mortalities after 48 h at 5 concentrations (0.01953125, 0.0390625, 0.078125, 0.15625 and 0.3125 mg/L) were not significantly difference compared with control, but the mortalities of accumulated before pupation at the rest concentrations were significantly lower than control, except the lowest concentration (0.01953125 mg/L). Similarly, the pupation rates at the rest concentrations were also significantly lower than control, except the lowest concentration. Besides that, the adult emergence rates at all tested concentrations were significantly lower than control, and expressed the higher the concentration, the more significant difference. However, the pupal weight between all the treatment groups and the control group were not significantly different.