The resistance to and the effects of synergists on the toxicity of six insecticides in Diaeretiella rapae (M’Intosh) (Hymenoptera: Aphidiidae), a parasitoid of vegetable aphid collected in Jianxin at Fuzhou-City, Fujian, China, were studied. In comparison with susceptible F₂₁ progeny, the resistance ratios in resistant F₀ parents were 27.6 for methamidophos, 20.8 for fipronil, 47.5 for avermectin, 3.3 for fenvalerate, 4.5 for cypermethrin, and 74.7 for imidacloprid. Piperonyl butoxide (PB), triphenyl phosphate (TPP), and diethyl maleate (DEM) were chosen to be applied in susceptible F₂₁ progeny, as well as in resistant F₁₁ progeny and F₀ parents. Significant synergistic effects on the toxicity of the six insecticides were found by using PB, TPP, and DEM in F₀ parents; on methamidophos, avermectin, and imidacloprid by PB, TPP, and DEM in F₁₁ progeny; on fipronil by PB and DEM in F₁₁ progeny; and on fenvalerate and cypermethrin by PB in F₁₁ progeny. PB also showed significant synergism on the six insecticides in susceptible F₂₁ progeny, although the synergism was far less in F₂₁ progeny than those in resistant F₀ parents. TPP and DEM showed little or no synergistic effects on the toxicity of the six insecticides in F₂₁ progeny. Compared with TPP and DEM, the highest synergistic ratios of PB for methamidophos, fipronil, avermectin, fenvalerate, cypermethrin, and imidacloprid were observed in F₀ parents, and F₁₁ and F₂₁ progeny. The resistance levels to methamidophos, fipronil, avermectin, fenvalerate, and cypermethrin could be inhibited strongly by applying PB in F₀ parents. From the results, oxidative degradation is believed to play a critical role in resistance to methamidophos, fipronil, avermectin, fenvalerate, and cypermethrin in D. rapae. To a lesser extent, hydrolytic reactions also were partially involved in the resistance to these five insecticides by using the synergists PB, TPP, and DEM. However, although high synergism of PB, TPP, and DEM on imidacloprid was found, the resistance levels to imidacloprid remained high in the presence of PB, TPP, and DEM. The mediated detoxification of oxidative degradation and hydrolytic reactions was thought to be involved in the resistance to imidacloprid in F₀ parents.