Although silicon (Si) is not known as an essential plant nutrient but has several beneficial effects in plant. Soluble Si may activate defense mechanisms in crops against biotic stresses including root-rot diseases. The Phytophthora melonis is one of the most important root-rot diseases and the main agent responsible of green death in cucumber. We hypothesized that Si affects physiological and biological mechanisms in cucumber plants to improve the crop tolerance to disease stress. Thus, the aim of this study was to investigate the influence of Si on the tolerance of two cucumber genotypes ( Cucumis sativus L. cvs. Dominus GRC and Super Dominus) to Phtophthora melonis infection. In a hydroponics experiment, two cucumber cultivars were exposed to three Si levels (0, 100, and 200 mg L -1 Si) and two Phytophthora melonis inoculums levels (with and without inoculation of P. melonis ). Shoot and root dry matter yield, root membrane permeability, reactive sulfhydryl group concentration, activity of catalase (CAT) and ascorbate eroxidase (APX) in the roots, and concentration of zinc (Zn), iron (Fe), manganese (Mn) and silicon (Si) in the roots and shoots were determined. After plant harvesting, the severity of P. melonis infection was assessed in terms of the number of roots with at least one pathogen lesion. Supplement of Si in the nutrient solution increased the Si concentration in the cucumber tissues. The root length and root and shoot dry weight of cucumber plants was significantly ( P 0.05) greater in the presence of Si to the compared with Si free treatments. In both cucumber cultivars, in the presence of P. melonis infection, Si nutrition significantly ( P 0.05) decreased the root membrane permeability and increased the activities of CAT and APX in the root. There was no significant difference in the reactive sulfhydryl group content between the infected and non-infected cucumber plants. Silicon nutrition significantly ( P 0.05) decreased the disease-induced injurious effects on the cucumber roots. Zinc and Mn concentration in the roots decreased but the roots and shoots concentration of Fe increased as a result of Si nutrition. Application of Si had a positive effect on the tolerance of cucumber to P. melonis root-rot disease. The results showed that Si addition to the nutrient solution of cucumber may reduce the membrane oxidative damage induced by fungal infection and thus, improve crop tolerance to disease. Key words: Silicon, Phytophthora melonis infection, Cucumber, Plant tolerance