Abstarct: The quantity and quality of Agricultural goods is prone to huge losses due to the attack of various pathogens, e.g. viruses, fungi, nematodes and bacteria. Fusarium oxysporum is one of the major cucurbits pathogens which mostly damages the plant roots. There are several strategies to control this pathogen. Potassium phosphite is a systemic fungicide against soil and aerialbacterial and fungal pathogens which has preventative, curative and immunization effects. This study was conducted to study changes in growth and morphologic traits, and enzymatic, biochemical and molecular resposses (exxpression of pgip1, defensin, chitinase and beta 1,3-glucanase genes) of Potassium phosphite-treated cucumber plants against Fusarium wilt of cucumber root and stem (Fusarium oxysporum f.sp radicis-cucumerinum). Results revealed that compared to control plants, fresh weight, chlorophyll content, the activity of defense enzymes, e.g., Guaiacol peroxidase (GPX), Catalase (CAT), superoxide dismutase (SOD) and mextabolite Hydrogen peroxide (H2O2) were increased in the tissues of the treated plants compared to controls. The highest CAT content was recorded for 4 gr/L Potassium phosphite at 5 days post-inoculation which revealed a 2.81 fold increase compared to control. The highest concentration of GPX and SOD were recorded for 4 gr/L Potassium phosphite 3 days post inoculation which showed 1.45 and 2.21 fold increase for GPX and SOD, respectively. In 4 gr/L Potassium phosphite treaterment, there was a harmony between H2O2 content and activities of CTA, GPX and SOD in plant tissues, while MDA accumulation was reduced which can be attributed to reduced tissue damage and improved disease control. Gene exxpression analysis revealed a 16.3 fold increase for pgip1 gene at 4 gr/L Potassium phosphite, 45.7 fold increase for Defensin gene at 1 gr/L Potassium phosphite, 91.2 fold increase for Chitinase gene in pathogen-inoculated plants and 48.6 fold increase for 1,3-beta glucanase gene in pathogen-inoculated plants, compared to control plants.