Abstract: To clarify the effects of low dose spinetoram on the activitiy of detoxifying enzymes in Plutella xylostella (L.), the toxicities of spinetoram and spinosad to susceptible population of P. xylostella were assayed using leaf dipping method. And the changing dynamics of detoxifying enzymes of carboxylesterase, glutathione S-transferase and mixed-functional oxidase were measured after exposure to LC25 and LC50 doses of spinetoram at 6, 12, 24, 48 and 72 h, respectively. The results showed that the spinetoram had higher toxicity to P. xylostella than that of spinosad. The 48 h LC25 and LC50 of spinetoram were 0.018 and 0.048 mg/L, respectively. After treatment with low concentrations spinetoram, a large fluctuation of the CarE was observed. CarE activity in the treatment strain were higher than those in the untreated control strain after treatment 6-24 h, and showed tendency of first increasing and then decreasing. Whereas, from 24 h to 72 h, CarE activity was lower than that, and with a certain amount of time effect on P. xylostella. GST had obviously induced effect. And GST activity was higher than that of the control group. MFOs activities had been distinctly inhibited which were significantly lower than that of the control group, whereas they were similar at 48 h. These results indicated that GST may be the major detoxifying enzyme for metabolizing spinetoram in P. xylostella.