The present study was undertaken to analyze the changes in the physicochemical characteristics and antioxidant activity in acorn (Quercus acutissima) under various temperature treatments: A (140°C), B (150°C), C (160°C), D (170°C), and E (180°C). Each treatment sample was extracted with hot water for 4 h at 60°C. The pH of all heat-treated acorn extracts was significantly lower than that of the control, unheated acorn extract, and the pH was inversely proportional to the treatment temperature (p < 0.05). The sugar content of the acorn extract was significantly higher after heat treatment than that of the control (p < 0.05); however, it was higher than the control only in the groups below 160°C. Total polyphenol content was the highest in the group treated to the lowest temperature (A: 140°C), at 31.59 ± 0.21 mg GAE/g (p < 0.05). Antioxidant activity was influenced in a dose dependent manner (p < 0.05) being 31.03-95.99% for DPPH radical scavenging activity and 20.49-99.51% for ABTS radical scavenging activity in doses of 0.12-1.00 mg/mL of heat treated acorn extract. In conclusion, heat treatment of acorn fruits increased sugar content and antioxidant content as well as activity of acorn extracts, especially under lower heat treatment conditions.