The synergism of tea saponin on Tirpterygium wilfordii EC against larvae of geometrid Ectropis obliqua Prout was determined. The results indicated that the toxicity of tea saponin to the 2nd instar larvae of E. oblique was not obvious. When treated with leaves that had dipped in tea saponin solution at the concentration of 1,2,4 mg/ml respectively, the corrected mortality was only 27.03% ~ 37.84% on the 7th day after treatment. Toxicity test for T. wilfordii EC against the 2^nd larvae of E. obliqua was studied at the concentration of 0.8, 1.6, 3.2, 6.4 and 12.8 mg/ml respectively. On the 7th day after treatment, the corrected mortality was 27.27%, 33.78%, 40.54%, 56.76% and 63.51% respectively. There were significant synergistic actions on T. wilfordii EC with tea saponin against larvae of E. obliqua. Tea saponin at concentration of 4 mg/ml was added to T. wilfordii EC of 0.8 mg/ml,the corrected mortality of larvae of E. obliqua was significantly increased compared with tea saponin added at the concentration of 1 mg/ml, while no significant differences were found between tea saponin added at the concentration of 2 mg/ml and 4 mg/ml. Treated with T. wilfordii EC mixed with tea saponin of 2 mg/ml, the co-toxicity coefficients for the 2nd instar larvae of E. oblique on 1st, 3rd, 5th, 7th day after treatment, were 667.68, 981.61, 1 706.15 and 1 209.76 respectively. It means that the toxicities increased 5.68~11.10 times, and the amount of T. wilfordii EC decreased over 85% to reach the same effectiveness. The toxicities increased 1.00~6.91 times for the 4th instar larvae of E. obliqua, and the amount of T. wilfordii EC decreased over 50%.