Abstract: In a stuy on the pathogenesis of Pythium aphanidermatum to Lolium perenne, the chlorophyll content, β-carotene content, PSⅡ, the MDA content, superoxid anion radical content, hill activity, root activity and relative water content were examined. Chlorophyll,β-carotene was destroyed, F_v/F_m, and F_m became lower but the F_o of NuspeedⅡwas not changed, while the F_o of Inspire became higher. The MDA content and superoxid anion radical content became higher and the hill activity became lower when the concentration of toxin increased, and root activity and relative water content were also affected.