Abstract: Photosynthetic capacity directly determines the productive capacity of plant, and it is often affected by intrinsic factors of leaves and environmental conditions. In this study, following the day scale of leaf age, the changes of photosynthetic parameters of Quercus mongolica with leaf age and  their relationships with leaf functional traits from leaf budding were analyzed. We found that during the first few days after leaf budbreak, leaf respiration rate of Q. mongolica was higher than photosynthetic rate, hence the value of net photosynthetic rate was negative. The maximum net photosynthetic rate (P_max), the maximum net photosynthetic rate when CO₂ was saturated (J_max), and the maximum Rubisco catalytic reaction rate (V_cmax) were influenced more greatly by environmental factors (precipitation, soil moisture) during the middlegrowth period than in other periods. During the whole early growth season, the correlations among photosynthetic parameters and chlorophyll content and leaf area were extremely significantly positive (P<0.01), while the relationships between photosynthetic parameters and specific leaf weight were negatively correlated in leaf budding period of the early growth season, positively correlated in leafexpansion period of the early growth season, and negatively correlated after leaf fully expanded in the early growth season. In the middlegrowth season, there was no significant correlation between photosynthetic parameters and leaf functional traits. At the end of the growth season, the correlations between photosynthetic parameters and functional traits were markedly positive (P<0.05). Our study indicated that it was necessary to differentiate leaf photosynthetic capacities and their physiological and biochemical mechanisms following different growth stages or periods.