Abstract: The synergetic effect of β-cypermethrin and Bacillus thuringiensis(Bt) was studied by bioassay through Bt pre-treatment before β-cypermethrin application or mixing together with β-cypermethrin in the lab.At the same time, the effect of β-cypermethrin on the growth-curve of Bt was determined.The results showed that the LC50 values of β-cypermethrin against 2nd larvae of Hacillus cunea,pre-treated by Bt S19 at 1×106cell/mL one day before the treatment of β-cypermethrin or not,were 0.76 and 1.28 mg/L in 48 h respectively.And the LT50 values of the two treatments were 9.23 h and 30.67 h,respectively,when the concentration of β-cypermethrin was 2.25 mg/L.These results demonstrated that there were synergized action between Bt and β-cypermethrin by Bt pre-treatment.When β-cypermethrin mixed together with Bt,the co-toxicity coefficient (CTC) was 122.5 as the concentration of β-cypermethrin and Bt S19 was 6 mg/L and 1×106cell/mL respectively.However,the mixture showed some antagonism as the concentration of β-cypermethrin increased.The growth-curve experiment of Bt revealed that β-cypermethrin delayed the exponential phases of Bt,and the effect related with the concentration of β-cypermethrin positively,which suggested that β-cypermethrin might affect the germination and growth of Bt.All these experiments indicated that Bt pre-treatment before β- cypermethrin application was more efficiency than the mixture of β-cypermethrin and Bt against H.cunea.