Abstract: To investigate the bioactive and the insecticidal mechanism of tea saponin on pest, toxicity effect of tea saponin on eggs and larvae of Ectropis oblique was determined. Toxicity test for eggs of E. oblique was conducted at the concentration of 0.1, 0.2, 0.4, 0.8, 1, 2, 4, 8 mg/mL, respectively, and the corresponding hatch rate was 21.9%, 16.2%, 18.2%, 11.7%, 7.7%, 6.2%, 5.4%, 5.9%, respectively. This result indicated that toxicity effect on eggs of E. oblique increased with the increase of tea saponin concentration. Leaves were dipped in tea saponin solution at the concentration of 1, 2, 4, 8 mg/mL, respectively, and then fed the third instar larvae of E. oblique. The results showed that all the larvae survived, meaning that tea saponin had no significant toxicity on the third instar larvae of E. oblique. Antifeeding effect of tea saponin on the third instar larvae was also determined. The results indicated that tea saponin had antifeeding effect on the third instar larvae and the effect increased with the increase of the concentration. When the concentration of tea saponin was at 4, 8 mg/mL, the antifeeding rate was 80.0%, 85.7%, respectively. Body length and weight of the third, fourth and fifth instar larvae which were treated by tea saponin had no significant difference compared with that of the control group, indicating that feeding tea saponin at or below the concentration of 8 mg/mL had no significant effect on the growth and development of E. oblique.