Abstract: In order to clarify the synergistic interaction of pyraclostrobin and difenoconazole against Alternaria solani,the toxicity of pyraclostrobin,difenoconazole and their mixtures at different ratios against Alternaria solani was determined by mycelial growth rate test and conidial germination assay,the joint-toxicity of the two fungicides was assessed with Wadley formula,and the efficacy in controlling potato early blight and the effect of their synergistic mixtures on potato yield was assessed through the field trials.The result showed that the mixtures of 1∶1,2∶1,3∶1,4∶1,5∶1,1∶2 and 1∶5 exhibited synergistic interaction against the mycelial growth of A.solani,the synergistic effects of the 1∶2 mixture was most obvious with synergistic ratio (SR) of 2.05,and the 1∶2 mixture exhibited additive effect against conidial germination with SR of 1.29.The result of field trials on controlling efficacy showed that the tank mixture (1∶2, W/W) exhibited significantly higher efficacy in controlling potato early blight than single agent alone with the same rate of application and the other common fungicides,and the mixture gave 46.8% of the yield increase ratio of potato,significantly higher than that given by the fungicides as control.