Abstract: Toxicity of metalaxyl-M, metalaxyl and their mixtures to Phytophthora drechsleri were determined by means of growth rate, respectively. Effect of metalaxyl-M and metalaxyl on hypha growth, sporangium germination, swarm sporangium formation were examined by the ways of liquid-solid white lupine media mixed with fungicide of different concentration. The result indicated that the EC50 of metalaxyl-M and metalaxyl were 0.129 μg/mL and 0.427 μg/mL . The common toxicity index of mixture 1 containing metalaxyl-M and mancozeb (5∶48, m/m) and mixture 2 containing metalaxyl and mancozeb (5∶48, m/m) were 101.6 and 95.9, respectively. Metalaxyl-M and mixture 1 showed more effective toxicity to Phytophthora drechsleri than that of metalaxyl and mixture 2. Metalaxyl-M had higher inhibitory action on hypha growth and sporangium formation than that of metalaxyl under the same concentration. When the concentration was 0.3~0.7 μg/mL and 30.0~70.0 μg/mL respectively, the inhibition rate of metalaxyl-M and metalaxyl to the growth of hypha were 39.28%~84.39% and 28.36%~66.56%, to sporangium formation were 42.7%~98.3% and 25.1%~89.5%, but metalaxyl-M or metalaxyl had no inhibitory activity to sporangium germination. Effect of fungicides on the cell membrance potential of Phytophthora drechsleri was tested by the conductivity method, which reveal that metalaxyl-M made a significant infection on cell membrance than that of metalaxyl.