Density is one of the important factors that affects the growth of forests, especially plantations. Tree crowns are places where forest ecosystems exchange energy and materials with other systems. The response of the growth of trees and tree crowns to forest density can be considered as a phenomenon of adaptation by organisms to the environment. Studying the response of tree growth to density effect is an important topic in ecology and silviculture. The objective of this study was to investigate the relationship between tree diameter at breast height (DBH), tree height, crown width and the stand density in even-aged Platycladus orientalis plantation. In total 23 plots with 8 different densities were etablished in Beijing mountain area, China in 2009. The densities of stands varied between 755 and 4577 trees/hm2. Two type of plots with an area of 20 m×20 m or 15 m×15 m were created, Average ages of selected stands were between 24 and 40 years. We referred to the crown investigation indexes developed by USDA, Forest Service during the investigation. The crown growth indexes were grouped into two-dimensional indexes and three-dimensional indexes. Results of the investigation were shown as following: (1) Stand density affected the growth indexes significantly. The growth of the average DBH, average height, and average crown width all decreased accompany the increase of density. The effect was particularly obvious when the density was under 3000 trees/ hm2. When the density was above 3000 trees/ hm2, the decrease of the growth indexes slowed. We built alleometric models to predict DBH, tree height, and crown width from the density. The format of the model was y=axb, the correlation index (R2) were 0.990, 0.955 and 0.891 for three models. (2) The increase of density affected the shape of tree crowns greatly. The crowns were inhibited significantly in both horizontal and vertical directions. The shape of tree crowns appeared to change from plump to narrow. (3) Two-dimensional indexes and three-dimensional indexes were correlated to tree densities. The result showed that the crown length, crown ratio, crown production efficiency, crown density, composite crown surface area, composite crown volume were negatively correlated with the tree density, while the crown fullness ratio, crown production efficiency were positively correlated with the tree density. All those indicated that the increase of stand density could inhibit the growth of tree crowns significantly. (4) Branches were analyzed to study the effects of tree densities on the length of branch, average growth increment of branch volume, and consecutive annual growth increment of branch volume. The increment of length of branch was not significantly different from that of the branch volume in young stands. The increment of length of branch did not change significantly as the tree density has been changed. The growth of volume of branch responded negatively to the tree densities. This might be caused by the complex environment. (5) Crown width is the most easy-to-measure index of tree crowns during forest inventory and other indexes can be calculated from it. A model was constructed to predict the crown width from DBH, crown ratio, and stand density. After considering the multicollinearity of independent variables, the final model was only based on DBH. The DBH-CW model could predict the growth of crown width of young Platycladus orientalis plantations well (R2=0.96).