The effects of a diamondback moth-resistant Chinese cabbage (Brassica campestris), expressing the insecticidal protein Cry1Ac toxin derived from Bacillus thuringiensis, on the non-target herbivore Mamestra brassicae and its parasitoid wasp Microplitis mediator, were determined in laboratory studies. When reared for their whole lifespan on the transgenic cabbage, significantly higher larval mortality and reduced pupation rate and pupal weight were observed but no apparent effects were found in the developmental times, larval weight and head width of M. brassicae. In contrast, when M. mediator wasps were reared with the M. brassicae larvae fed with the transgenic Chinese cabbage, no significant changes were observed in all the examined indices including parasitization rate, time to cocoon formation, cocoon weights, adult emergence time, emergence rate and adult longevity. The ELISA test using Cry1Ac-anibody implied that the Cry toxin consumed by M. brassicae larvae is not likely exposed to its endoparasitoid M. mediator although the Cry toxin was detected in the whole body of M. brassicae larva from 6 to 96 post feeding hours. Taken together, the transgenic Chinese cabbage appears to affect the survival and pupation rates of M. brassicae but little to cause adverse effects on its parasitoid wasp M. mediator.