Abstract: Objective To explore the toxicity and its assessment methods of pesticide mixtures.Methods The Daphnia pulex were exposed to carbaryl, piperonyl butoxide as well as the mixtures of the two pesticides and the activity inhibition rates of Daphnia pulex were detected.Then based on the concentration-response curves of individual compound and interaction coefficient, two models including concentration addition (CA) and independent action (IA) were employed to predict the joint toxicity of the mixtures.The concentration-response curves predicted by CA and IA were compared with that of based on the experimental data.Results The data of acute toxicity experiment of carbaryl, piperonyl butoxide and the mixture could be fitted by Weibull equation, and the correlation coefficients(R²) were 0.89005, 0.92834, and 0.944 41.The values of median effective concentration(EC₅₀) obtained by the equations were 0.0714, 5.7147, and 2.968 μmol/L.A synergistic interaction between carbaryl and piperonyl butoxide was observed.After introducing K-functions, the fitted curve predicted by IA models was more approximate to the fitting curve based on the experimental data in the assessment of joint toxicity of the two pesticides.Conclusion Toxic effects of pesticide mixture can be predicted by models, and the interactions of the pesticide mixture can be modified by K-function.