Abstract: Mulberry leaves sprayed with cypermethrin were continuously fed to 2rd instar silkworms till 3rd instar. The study on residual effect of the pesticide on mulberry indicated that leaves sprayed with 1.08 μg/L showed toxicity after 63 days, while 108.00 μg/L sprayed plants new sprouted leaves did not show toxicity even after 10 days. The effect of different concentrations of cypermethrin fed to 3rd instar larvae and its influence on larval mortality, larval weight during moult, cocoon weight, and shell ratio showed the mortality of silkworms and larval weight during moult fed with 0.11 ng/L cypermethrin were significantly(P＜0.05) increase and decreased respectively. The silkworms fed with 1.08 ng/L cypermethrin, the mortality of silkworms, and larval weight during moult and cocoon weight was significantly (P＜0.01) increase and decreased respectively. The influence of 10.80 ng/L cypermethrin on cocoon shell ratio was also significantly (P＜0.01) decreased.