We studied effects of experimental warming on seasonal change in leaf traits and temperature response of photosynthesis in canopy leaves of Quercus serrata in a cool-temperate forest. On the top of three Quercus serrata trees a large branch was enclosed in a 2×2×2 meter acryl box (open top canopy chamber : OTCC). The OTCCs increased temperature by about 5℃ at midday on a sunny day. We determined photosynthetic rates of leaves in OTCCs and those in the control monthly from June to October in 2011. Photosynthetic rate at 20℃ was not changed by the temperature treatment. However, photosynthetic rate and carboxylation capacity (V_cmax) at 25℃ tended to be higher in leaves subjected to warming. The activation energy of V_cmax (E_av) was higher at warming experiment, and optimal temperature of photosynthesis was positively correlated with E_av across month and treatment. We conclude that canopy leaves of Quercus serrata increased optimal temperature of photosynthesis mainly through increase in E_av and maximized the photosynthesis under the growing temperature.