Abstract: In this paper, the importance of niche overlap to plant ecology is expounded. It is pointed out that the measurement of niche overlap between plant species is distinct from that between animal species.The author suggests that ecological factor gradient should be replaced by community gradient and the interval of ecological distance may be used as weighting factor in the formula for measuring niche overlap. Thus measurement of overlap can be simplified. Taking the herbs growing under the forests of the Mahan Mountain as an example, the author measured the overlaps between species on two different conditions respectively, i. e. first on the gradient of light intensity, soil moisture and organic matter, and then on the community gradient. Both the results were analysed. Finally, the difference between partial overlap and total overlap and that between quadrats and resource states were discussed.