Abstarct: Fungal diseases are one of the main causes of damage & reduced yield of agricultural products. The production of greenhouse cucumber (Cucumis sativus L.) have been extensively grown in Iran during the past decades. Damping off caused by various Phytophthora species is one of the most important diseased affecting cucumber culture & there are few controlling measures to reduce its damages. In this study, chitosan & potassium phosphite were used as potential resistance inducers & their effect on reducing the damages caused by Phytophthora capsici in cucumber plants was investigated. Different physiological, biochemical & mextabolic alterations along with changes in exxpression of glutathione peroxidase (Gpx), tomatine-like protein (TLP), Respiratory burst oxidase homolog (RBOH), Hypersensitive-induced respose reaction (HIR), Allene oxide synthase, chloroplastic-like (HPL) & Endo chitinase (PR-4) genes were studied at different time-courses after pathogen inoculation. Results showed that in comparison with the control plants, the content of chlorophyll, proline & activity of protective enzymes i.e. ascorbate peroxidase, superoxide dismutase, guaiac peroxidase, catalase & in accumulation of hydrogen peroxide & malondialdehyde mextabolites were significantly changed in different treatments .The highest amount of ascorbate peroxidase enzyme (21/93 U mg-1protein) was recorded in treatment with 200 mg / L chitosan 14 days after infection. Also, the highest activity of guaiacol peroxidase (41/99 U mg-1protein) & catalase (150/34 U mg-1protein) enzymes was observed in treatments including concomitant application of potassium phosphate & chitosan with a concentration of 200 mg / L on the second day after inoculation & time of inoculation, respectively. The highest accumulation of hydrogen peroxide (2/19 U mg-1protein) & malondialdehyde (25/57 U mg-1protein) was recorded 28 days after inoculation in treatment including concomitant application of potassium phosphate & chitosan with a concentration of 200 mg / L. Gene exxpression analysis showed that the highest exxpression of RBOH, HIR, HLP and PR-4 genes was reported in potassium phosphate and chitosan simultaneously in the first day after inoculation of the disease, and their exxpression increased by 35.54, 306/136, 75/6 and 24/259 times the control treatment. The use of potassium phosphate & chitosan by increasing activity of ascorbate peroxidase & catalase protective enzymes, & also induction of exxpression of the resistance genes were able to induce resistance in cucumber plant that was under stress of the fungus causing damping-off disease.