Abstract: By using 1961-1990 summer (June-July-August) rainfall data from 66 stations located at more than 2000m above sea level on the central and eastern Qinghai-Xizang Plateau,the temporal evolutions and spatial structures of the summer rainfall are revealed through the empirical orthogonal function (EOF) analysis. It is found that the dominant spatial pattern of the summer rainfall variation over the Plateau is a seesaw between the southern and northern parts and the temporal variation of the dominant pattern is closely correlated with the North Atlantic Oscillation (NAO).A possible mechanism of the correlation is proposed on basis of statistical analyses with the geo-potential height and wind field data over the Northern Hemisphere. In a summer when the NAO index is low, the westerlies in mid-latitudes from eastern Atlantic to western Europe increases. Enhanced westerlies will bring about a strong flow around the Plateau when passing the Plateau and so the ridge or anticyclonic vorticity in the northern part and the trough or cyclonic vorticity are strengthened simultaneously. In this case, the summer rainfall is usually above the normal in the southern Plateau but below the normal in the northern Plateau. In a summer with a high NAO index, the above processes are opposite.