Backround: The antioxidant capacity of red beetroot has an important role in promoting human health and preventing degenerative diseases and cancer. Beetroot and colorful root vegetables are rich in phenol compounds with antioxidant properties. Results: In this study, changes in total phenolic and flavonoid amounts and antioxidant activities and phenolic constituents of red beets heated in oven, microwave (MW), and infrared (IR) systems were investigated. The total phenolic and flavonoid contents of the red beetroots heated in oven, MW, and IR were determined between 327.85 (IR) and 619.27 mg GAE/100 g (oven) to 674.03 (IR) and 1089.24 mg/100 g (MW), respectively. Also, the antioxidant activity values of dehydrated red beet samples varied between 0.02 (IR) and 4.73 mmol/kg (Oven). The values of total phenolic, total flavonoid values, and antioxidant activities of the red beet samples were most affected by infrared. Gallic acid and 3,4-dihydroxybenzoic acid amounts of red beet samples were identified between 47.54 (Oven) and 438.65 mg/100 g (MW) to 15.98 (IR) and 45.44 mg/100 g (MW), respectively. Also, the highest p-coumaric acid (8.50 mg/100 g), ferulic acid (6.26 mg/100 g), resveratrol (2.55 mg/100 g), quercetin (4.69 mg/1000 g), cinnamic acid (0.21 mg/100 g), and kaempferol (0.64 mg/100 g) were established in red beets dehydrated in MW system. Conclusion: When the red beet is desired to be rich in total phenolic and total flavonoid components, conventional and microwave oven cooking should be preferred, respectively. The phenolic components of the red beets dehydrated in the microwave were found to be high (except caffeic acid) compared with oven and IR.