Abstract: The effects of transgenic Bt cotton on the parasitoid in tri-trophic system involving Sukang 310, Sylepta derogata and Apanteles ruficrus have been investigated. The negative effects of Bt cotton (Sukang 310) on the development of parasitoid ( Apanteles ruficrus ) were found in the fields. The population of Apanteles ruficrus descended significantly. The parasite ratio in the experiment is lower than that of the control.