In order to understand how morphological characteristics of Quercus Mongolica seedlings respond to variations in light conditions, we measured differences in dry mass per plant, height growth, root-to-shoot ratio, specific leaf area (SLA), and the volume of apical bud across both seedlings growing under forest canopy and open field. Our data indicated that light conditions significantly influenced the morphological characteristics of Quercus Mongolica seedlings. The dry mass per plant in open-field seedlings was 4.48 times as that in undergrowth seedlings. There was a larger difference in root mass as compared to shoot mass, in which the root mass of open-field seedlings was 6.23 times as that of undergrowth seedlings. The root-shoot ratio was 2.70 and 1.11 for open-field and undergrowth seedlings, respectively. The SLA was lower in the open-field site (139.55cm2/g) than in the undergrowth site (284.94cm2/g). The area of first-flush leaves was lower in the open-field site than in the undergrowth site, but the total leaf area including first- and second-flush leaves of a plant indicated an opposite pattern. Although the second-flush leaf growth led to an increase of leaf area, there was no difference in total dry mass per plant between seedlings with and without second-flush leaf growth, suggesting that second-flush leaves did not contribute to biomass accumulation of a plant. The height growth was significantly lower in open-field seedlings than in undergrowth seedlings, but the basal stem diameter showed an opposite pattern. The length and diameter of roots and the volume of apical buds were typically larger in the open-field site than in the undergrowth site. The second-flush leaf growth led to a decrease in the volume of apical buds of open-field seedlings, which might affect the height growth of next year. We concluded that Quercus Mongolica seedlings plastically adapt to different light conditions through changes in morphological characteristics to maximize carbon gain.