Abstract: Jatropherol I(JaI), a diterpene separated from Jatropha curcas seed oil was used to evaluate its toxicity to 3rd-instar silkworm, Bombys mori L., as well as its effects on midguts tissue and proteinases activity of midguts. The results showed that JaI had no contact toxicity but strong stomach toxicity to 3rd-instar silkworm. The LC50 value were 0.579,0.220 and 0.158 mg/mL respectively at 48, 72 and 120 h after ingestion of JaI. JaI had a significant antifeedant effect to silkworm with AFC50 of 0.160 and 0.145 mg/mL, respectively at 24 h and 48 h. A strong inhibition with obvious dose and time dependent effect was observed in JaI on the growth of silkworm. The inhibitory effect of JaI increased with time and concentration increased. After silkworm was fed with JaI treated mulberry leaves, proteinases activity in midguts reduced with time increased. The activity of proteinases reached the minimum at 48 h after treatment, which was only 32.94%,43.35%,71.45% of the control at 0.25,0.125 and 0.062 5 mg/mL, respectively. Tissue structure of midguts in silkworm exposed to JaI was examined by histological method. Damages in epithelial cells and disintegration of fat body were observed. It was suggested that damages to insect digestion system induced by JaI might be an important toxicological mechanism of JaI to silkworm.