The physiological and biochemical mechanism of prevention for cucumber downy mildew by high temperature treatment were studied using cucumber susceptible variety ‘Changchun mici’.Cucumber seedlings were potted in greenhouse.(1)The best effect of prevention reached after seedlings inoculated with Pseudoperonospora cubensis 8 h and then treated for 90 min at 45℃.(2)Compared with seedlings inoculated with P.cubensis,chlorophyll contents significantly increased and MDA content significantly decreased by high temperature treatment after inoculation.Compared with control,the activity of chitinase significantly increased by high temperature treatment after inoculation,and changes in chitinase activity was further proved by western blotting.(3)A 28 kD protein was induced after inoculation,and then decreased its expression by high temperature treatment.This indicated that high temperature treatment not only could kill much pathogens but also induce resistance against downy mildew.