Phosphite-based fungicides are a group of chemicals that are directly harmless to the environment and have a direct fungicide effect, while also indirectly stimulating the plant immune response. Potato late blight is one of the most destructive potatoes diseases globally, causing many morphological and physiological changes in potato leaves and tubers. In current experiment, the potato plants with three-time spray of potassium phosphite (KPhi) on potato leaves, following three months in storage, and inoculated with late blight were investigated. This study examined the possible role of KPhi in delays of late blight invasion and physiological responses including the reactive oxygen species (ROS) metabolism in potato tubers after challenging with late blight pathogen. Samples were collected at 0, 24, 48, 72 and 96 hours of time points for evaluations. Antioxidant enzymes such as superoxidase dismutase (SOD), peroxidase (POD) and catalase (CAT), in addition content of metabolites such as phytoalexin and phenolic after infection with pathogen was investigated. Meanwhile, KPhi treatment resulted in plant tolerance with improved resistance, increased antioxidant enzymes activities and non-enzymatic compounds compare to untreated plants under similar stress. The results of this study indicate that the role of KPhi reduces the adverse effects of pathogens, suppress potato late blight and improves antioxidant enzymes activities and non-enzymatic compounds in potato tubers.