Abstract: The source area of Lancangjiang(Mekong River)is situated in the south of Qinghai,China,between latitude 31°30′～33°53′N and longitude 92°35′～97°35′E.The area at an altitudinal range of 3500 m to 5900 m.,with c.50590 km² in total area.Its climate is plateau-continental one.There are 996 species of native seed plants which belong to 61 families and 279 genera in the area.They are 43.59% of the total species and 54.92% of the total genera,67.78% of the total families in Qinghai,respectively.The floristic characteristics in native seed plants for the area are as follows:(1) Number of species are relatively rich and woody ones are poor.(2) At generic level,the flora is temperate in nature,dominated by North Temperate elements especially the typical ones from the warm and cold zone of Eurasia and the Alpine from Sino-Himalayan elements of the E.Asia.(3) Many genera and species come from the Heng-duan Mt.and the northeast plateau of Tibet are closely related to these of the Himalaya.(4) Floristic-geographically,it is a convergent and transitional region for the Qinghai-Tibetan Plateau Alpine Flora and the Hengduan Mt.Flora.The transitional nature of the flora is conspicuously.(5) It is a part of the Tangute Flora.(6) Floristically,it is one of the most active differentiate area in the flora of Qinghai or in the Tangute Area.