Abstract: Joint actions of 281 mixtures of pesticides whose co-toxicity coefficients were over 100 were recalculated with the formula of co-toxicity factor and the results showed that 29.18% of the mixtures had no synergism. According to the LC-P line of each of insecticide in the mixture and the formula of co-toxicity factor,several expected mortality and the region of additive action (expected mortality±20% expected mortality) were calculated,and according to observed mortality of mixture,the 95% confidence interval was calculated,LC-P line of expected mortality with the region of additive action and LC-P line of observed mortality with the 95% confidence interval were drawn,it was found that there was overlap between the region of additive action and 95% confidence interval and two lines crossed each other when co-toxicity coefficient was more than 100 and the co-toxicity factor was less than 20,which meant there was no significant differences between expected mortality and observed mortality,and that there was on or a few overlap between the region of additive action and 95% confidence interval when co-toxicity coefficient was more than 100 and the co-toxicity factor was more than 20 or co-toxicity coefficient was less than 100 and the co-toxicity factor was less than 20,which meant there was significant differences between expected mortality and observed mortality.So,in order to obtain a synergistic mixture of pesticides to pest,the co-toxicity coefficient must be more than 100 and the co-toxicity factor was more than 20,too.