Abstract: From the data of 178 sample plots of forest communities in Guizhou, the species diversity of these communities is studied in this paper. The determined indexes include the Simpson diversity indices and the speices richness and evenness of the communities.The determined results are as follows: Different vertical zonal bioclimatic condition, s produce different diversities in forest communities. For similar vertical zonal bioclimatic conditions, the substrate habitats are similar, and the diversities are also similar in different subtypes of forest vegetation, though the substrate habitats and diversities may be different. The diversity has variation in each of the sample plots from the same community type. The community individual diversity indices of different structures are not similar. The succession: tendencies of the communities are also different. The diversity of the second substorey is generally higher than that of the first substorey. The more a community approached its stage of climax, the higher its diversity index in same series of succession. There is close correlationship between the diversity index and the richness and evenness of a community, but between it and the individual total number of communities is nonrelativity. The diversity determined is considered significant for comparing and describing the structure and type of community, organization characteristic, habitat, succession and so on.