Abstract: Bioassay of the time-dynamic intensity (TDI) of toxicity and the dynamic strengthen coefficient (DSC) were carried on under laboratory condition.DSCⅠand Ⅱ of Bacillus thuringiensis (Bt) and acephate and their mixtures to larvae of diamond back moth (Plutella xylostella Linnaeus)were obtained.TDI values were all calculated as the basis of respective ladder-shaped area by LT-p line.The results showed that: ①The number of good mixture ratios for mother solution pairs (MSP) of two insecticides varied with pairs A and B.According to LT50 of MSP,the pairs A showed acephate was weaker than Bt,in contrast,stronger than Bt in the pairs B.There were less 2 good mixture from pair A and 4~7 good mixture from pair B among 9 mixture combinations,that means pair B is better than pair A.②Good mixture in various combinations was related to the pairing of "LT50 to LT50" of MSP.For example,the pairs of 67.3/792.5 and 43.6/473.1 with small average value of DSCⅡ,being 73.5 and 90.4,respectively.In contrast,the pairs of 80.57/90.36 and 23.76/45.31 showed larger average values,110.3 and 101.8.