Abstract: Selective and complete anti-feeding bioassays were used to determine the anti-feeding activity of Bacillus thuringiensis(Bt) to resistant and susceptible population of diamondback moth, Plutella xylostella (L.). Bt showed some anti-feeding activity to the third instars larvae of both populations and its activity increased significantly with the increase of its concentration. The susceptible population was more susceptible to the effect of Bt than the resistant population. Its AFC50 is significant lower than that of resistant population. The resistant population had lower instinct to avoiding damage than susceptible population on treated leaves. The result implied that the resistance of diamondback moth to Bt did not relate with behavior resistance mechanism.