This paper formulates a novel hypothesis of community diversity on the basis of rejecting the competitive exclusion principle. Since we accept the view that many species could occupy the same niche, local species diversity is considered to be controlled by four fundamental factors,which are, respectively,(Ⅰ)the number of niches in the community,(Ⅱ)the size of regional species-pool, (Ⅲ) species immigration rate, and(Ⅳ) species extinction rate. The hypothesis suggests that both regional biogeographic processes and local ecological processes will play an important role in determining the magnitude and pattern of community diversity.