To investigate climate evolution during the Miocene, especially during the Middle Miocene climate transition on the northeastern Tibetan Plateau, stable oxygen and carbon isotopes of carbonates from a 288-m-thick lacustrine-fluvial sediment sequence covering the period from 17.1 to 6.1 Ma from Tianshui Basin, China, were analyzed. The relatively low stable oxygen isotope values indicate the prevalence of wet climate conditions during the period of 17.1-13.6 Ma, an interval corresponding to the well-known Middle Miocene Climate Optimum. The interval between 13.6 and 11.0 Ma (i.e., the late Middle Miocene) is marked by a progressive increase in the delta O-18 values, indicative of a decrease in precipitation, probably linked to the expansion of the East Antarctic Ice Sheet and global cooling since about 14 Ma. The climate in the study area continued to get drier as shown by the enrichment of the heavy oxygen isotope from 11 Ma. We attribute these stepwise climatic changes as revealed by our carbonate delta O-18 record from the northeastern Tibetan Plateau to the sustained global cooling that may have reduced moist transport to Central Asia, which in turn led to a permanent aridification.