Abstract: The relation between sex expression and three oxidases, i.e.peroxidase, polyphenol oxidase and superoxide dismutase, in cucumber (Cucumis sativus L.) was studied by means of isozyme electrophoresis.The bands of oxidases were more in the gynoecious plants than the monoecious plants and the activity much higher.This difference of isozyme made it easy to distinguish gynoecious cucumber plant from a mixing population in early growth stage.In our experiment, the polyphenol oxidase in the first true leaf showed the best results.