The effect of heating at different temperatures (60, 80, 90, 110, and 130 °C) on the total phenolic content, antioxidant activity, and phenolic compounds present in plum and mahaleb fruits was investigated. The antioxidant activity values and total phenolic contents of fresh plum (93.82% measured by DPPH method, 787.79 mg gallic acid equivalent (GAE)/100 g dry weight determined by Folin method) and mahaleb fruits (81.80%, 634.47 mg GAE/100 g dry weight) were higher than plum and mahaleb fruits dried at different temperatures (p < 0.05). Generally, the heating process caused a reduction in both total phenolic content and antioxidant activity for plum and mahaleb. While (+)-catechin (92.62 mg/kg), 1,2-dihydroxybenzene (132.15 mg/kg), gallic acid (107.01 mg/kg), and 3,4-dihydroxybenzoic acid (74.59 mg/kg) are the key phenolic compounds in fresh plum, (+)-catechin, 3,4-dihydroxybenzoic acid, 1,2-dihydroxybenzene, and syringic and caffeic acids were the major phenolic compounds of mahaleb fruits. The polyphenol content of fruits and the class of phenolics present are significantly affected by heating temperature.