Crown structure and competition of red birch seedlings were studied after one year exposure to ambient (AC) and elevated (EC) CO₂ concentration under Low (LD) and high (HD) planting density. Crown width, crown depth, branch number, branch angle and length were measured and derivative indices such as crown surface area, crown volume, leaf number and distribution were also examined. Crown width, crown depth, crown surface area and crown volume were all stimulated by elevated CO₂ and reduced by elevated planting density. The results showed that the effect of elevated CO₂ was higher under low planting density than under high density and the effect of elevated planting density was greater under elevated than under ambient CO₂. LDcpa(leaf number per unit projected crown area), LDcv(leaf number per unit crown volume ) and branch angles from the bottom branch group of tree crown were negatively affected by both elevated CO₂ and planting density. Effect of elevated CO₂ on branch length was related to branch position on the main stem. In conclusion, elevated plant density could reduce the positive effect of elevated CO₂ and the addition of elevated CO₂ might alleviate the negative effect of elevated plant density on crown features. The smaller LDcpa indicated that leaves were less overlapped and self-shaded under elevated CO₂, and the lower LDcv suggests that leaves in crown under elevated CO₂ were less crowded than that under ambient CO₂. Thus conclusion could be drawn that competitive pressure in crown was prone to be alleviated but not intensified through the effect of elevated CO₂, due to the fast extending of crown width and crow depth resulted from stimulated growth.