Bacillus thuringiensis (Bt) is effective to control the diamondback moth, Plutella xylostella. However, its relative slow and unstable control efficacy limits its wide use by farmers. To facilitate pathogenic rate of Bt, a bacterial mixture technique has been developed in this study. Two entomopathogenic bacteria, Xenorhabdus nematophila (Xn) and Photorhabdus temperata temperata (Ptt), possess high immunosuppressive activity against several lepidopteran insects. The mixture treatments using Bt + Xn or Bt + Ptt significantly enhanced Bt pathogenicity in median lethal concentration and time. Though live Xn and Ptt bacterial cells gave significant effect on the pathogenicity, their 48 h culture broth after removing the bacterialcells still possessed the synergistic effect on the Bt pathogenicity. The larvae fed with the bacterial culture broth suffered significant immunosuppression in response bacterial to infection