Abstract: We studied the niche breadth and niche overlapping of the dominants in Platycladus orientalis forests in Yanshan Nature Reserve, Shanxi Province by applying Shannon-Wiener index and niche special and general overlapping indices (Petraitis indices). The results indicated that the niche breadth of P. orientalis was the largest (3.56) and Pinus bungeana the second (3.12) among all dominant species. For the niche breadth of shrub species, Vitex negundo var. heterophylla was the largest (3.43), Rosa xanthina and Lespedeza floribunda were tied for second (3.09), while for herbaceous species, Carex lanceolata (3.39) and Artemisia brachyloba (3.06) exceeded others. It was mainly because of their extensive adaptability to the environment. The relationship between niche breadth and distribution frequency of the species was positive (r=0.941, P<0.01), indicating that the lager the distribution area, the greater the ecological adaptability to the environment for a species, and thus a wider niche breadth could be. All dominant species were not extensively overlapping in the communities due to high environmental heterogeneity and mutual ecological complementation among species. A total of 1099 species-pairs had niche overlapping, accounting for 87.22% of the total 1260 species-pairs from 36 dominants. There was little niche overlapping between P. orientalis and other species, implying that P. orientalis is more competitive and extensively adaptive than the other species.