Under climate change, the escalating in temperature is considered a serious challenge threaten the agricultural production. This production includes food, feed and all other human needs. These higher temperatures represent heat stress, which cause physiological and biochemical damages in cultivated plants. Nano-selenium (nano-Se) has distinguished features enhancing the cultivated plants under stress. These features include the high bioactivity of nano-Se, which ameliorate the growth of different cultivated plants under different abiotic and biotic stresses such as salinity, drought, waterlogging and heat stress. This study was carried out to investigate the ameliorative role of nano-Se on two sensitive chrysanthemum cultivars under high temperature stress (i.e., 39.5, 40.6 and 41.6℃). The growth of these cultivars, plant antioxidant system and floral attributes were investigated under graded nano-Se doses (i.e., 50, 100, 150 and 200 mg nano-Se L-1) during two successive growth seasons. The foliar application of nano-Se improved heat tolerance in chrysanthemum by enhancing the activities of antioxidant enzymes including catalase and peroxidase (up to 150 mg nano-Se L-1) and decreasing polyphenol oxidase and electrolyte leakage up to 200 mg nano-Se L-1. The nano-Se-treated cut chrysanthemum flowers under heat stress indicated the positive effects of nan-Se on improving of flower quality and the economic value. These results may confirm the definite roles of nano-Se in management the heat stress-induced adverse effects on not only cut chrysanthemum flowers but also other crops under climate change.