Xizang (Tibet) is the main body of the "World Roof" -the Qinghai-Xizang Plateau. Although the continuous uplift of the Plateau and the global climatic changes had no distinct influence on the entire geomorphological framework notable paleogeographical evolution has been witnessed. According to C14 dating, the process can be divided into three stages. 1) early Holo-cene (10000-7500 years B.P.), when the environment became better, for instance, the retreat of glaciers, the increase of plants, the development of peat and marshes, and salt-lake precipitation in the interior of northern Xizang Plateau ; 2) middle Holocene (7500-3000 years B.P.) , when both Vegetation and peat-marshes were in the most favourable condition, the forest line occurred over the plateau surface in southern Xizang; glacier retreated to the present position; lakes were at a stage of relatively high water level; climate was warm and humid and human activities spread all over the plateau, 3) late Holocene (3000 years B.P.to the present), also known as neo-ice age. Three glacial advances are seen and natural environment deteriorates; vegetation on the plateau surface is becoming steppe and desert steppe while peat and marshes decline; lakes diminish, and most of the exterior lakes turn into interior ones on the Plateau. Furthermore, salt precipitation becomes stronger; ancient human beings migrate towards the lower parts due to the cold and dry climate, and the northern part of the N.Xizang Plateau became a depopulated area.Distinct paleogeographic regional diff erenciation is recognized in Xizang.The greatest environmental change has been witnessed in the high mountain and broad valley-lake basin region of Southern and central Xizang where the transformation of exterior lakes into interior ones, the development of peat-marshes and the advance and retreat of forest line mostly occur.