he Photosynthetic characteristics of Quercus mongolica were studies based on the measurement of leaves photosynthesis under artificial controlled conditions of light intensities and CO2 concentration in region of Changbai Mountain. For mature trees, the light compensation point (Lcp), light saturation point (Lsp) and apparent quantum efficiency (α) were 21μmol/m2/s, 1564μmol/m2/s and 0.045μmol/mol, respectively. For young trees, the values of Lcp, Lsp, α, and CO2 compensation point, CO2 saturation point were 29μmol/m2/s, 1581μmol/m2/s, 0.049μmol/mol, 73μmol/mol, 625μmol/mol, respectively. The photosynthesis assimilation of young trees was more active than mature trees. It suggested that the Quercus mongolica was intrinsically helophytic plant, but the use efficiency of light quantum was low in strong light condition. The results also showed that the diurnal variations of net photosynthetic rate of young trees assume a typical midday depression pattern. Strong light intensity and high VPD were the key influencing factors.