Abstract: Basing on the multivariate analysis—ordination, quantitative classification, and environmental interpretation of 163 plant community samples collected from Ngari, Xizang, the primary vegetation types, ecological gradients, and their quantitative relations with environmental factors of Ngari are given. The basic analysis contains the following three steps. 1) the two principal ecological gradients are brought to light by two vectors of ordination scores produced by detranded correspondence analysis (DCA); 2) the nonhierarchical and hierarchical classification systems of plant communities are produced from the 2-dimensional scatter polt of DCA ordination and two-way indicator species analysis (TWINSPAN), respectively; 3) environmental indexes (quantitative interpretation) of various vegetation types are given by the multivariate regression analysis which connects the ordination scores with environmental and geographical parameters. It is shown by the analysis that the vegetation types of Ngari and their distribution are mainly determined by the thermal and moisture gradients. The former could be expressed quantitatively with the mathematic expression of geographical parameters, the latter with the soil characteristics. The two gradients contain quite different vegetation types, species, and habitates, ranging from low montane warm desert to alpine periglacial vegetation, and from intrazonal bog and saline meadow to zonal plateau desert and steppe with their paricular position and ordination scores. That shows that the quantitative analysing method used here for handling the ecological data of plant communities with great ecological diversity is highly efficient.