The effects of heat treatments on the antioxidant activities of selected fruits and vegetables were investigated by heating at various temperatures (110, 120, 130, 140, and 150$^{circ}C$) for 2 hr. The examined fruits and vegetables included tomatoes (Lycopersicon esculentum), oriental melon (Cucumis melon var. makuwa), apples (Malus pumila Miller var. domestica Schneider), melon (Cucumis melon), watermelon (Citrullus vulgaris), and banana (Musa sapientum). The total polyphenol and total flavonoid contents of the juices from heated samples were quantified spectrophotometrically, and their antioxidant activities were determined using DPPH and ABTS radicals. As the heating temperature increased, antioxidant activity also increased. The highest total polyphenol content (2.80 mg/g) occurred in the oriental melon with heating at 150$^{circ}C$, and this value was 7 times higher than that of the untreated oriental melon (0.40 mg/g). The highest total flavonoid content (148.80 ${mu}$g/g) occurred in the melon heat treated at 150$^{circ}C$, and this value was 37 times higher than that of the untreated melon (4.54 ${mu}$g/g). DPPH radical-scavenging activity was the highest in the watermelon treated at 150$^{circ}C$ (84.37%, 0.50 mg/g), and this value was 40 times higher than that of the untreated watermelon. Finally, the highest ascorbic acid (AA) equivalent antioxidant capacity (AEAC) value (239.50 mg AA eq/g) was obtained in the watermelon heat treated at 150$^{circ}C$ for 2 hr (control = 18.35 mg AA eq/g).