Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is one of the most destructive insect pests of cruciferous crops. This study is to determine the effect of a combined treatment of Bacillus thuringiensis (Berliner) and Trichogramma ostriniae (Peng and Chen) (Hymenoptera: Trichogrammatidae) applied for an eco-friendly control of Diamondback moth, P. xylostella. Results of the investigation of the resistance ratio of four field populations of P. xylostella to B. thuringiensis shows that Hoengseong populations were highly resistance to the 17.1-fold of B. thuringiensis subsp. aizawai and to the 36.0-fold of B. thuringiensis subsp. kurstaki. B. thuringiensis was proven to give no detrimental effects on adult survival and eggs parasitism of the wasp based on contact toxicity test. Survival rates of P. xylostella were 31.67% and 14.17% on single treatment of T. ostriniae and B. thuringiensis, respectively. But, survival rate of P. xylostella was 1.07% on a combined treatment of egg parasitoid and B. thuringiensis. The combined treatment of both biological control agents significantly decreased the crops damage rate, though none of the biological control agents alone showed any significant control efficacy.