Abstract: Size structure analysis of Quercus wutaishanica (Liaodong oak) forest in a mountainous area of Beijing city indicates that while the oak forest is rather stable its regeneration mainly relies on stem base-sprouting. The size structure of populations of Acer mono, Ulmus macrocarpa, and Fraxinus rhynchophylla, the main associated species in the oak forests ,indicate that these are also able to co-exist the oak. The phenomenon of sprouting from the bases of the oak stems is very common in these forests. In addition to sprouting from stumps and bases of dead and aged individuals, new stems of seedlings and saplings also usually sprout basally. The size structure of the oak populations shows a clear bottleneck at the sapling stage (size class 2) during forest regeneration. However, through its sprouting ability, the number of stump-sprouting, size-class 3 individuals is large enough to make up the shortage of saplings and complete the regeneration of populations. It is as a result of this resprouting process that the forest is able to maintain its stability. On the other hand, the cohort of seedlings ,though limited in size, has ecological significance through its role in maintaining genetic diversity, the vigor of the oak population as whole and the stability of the communities in which it occurs as well as improving thee oak’s adapting ability to the environmental changes.