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The stable isotopic composition of precipitation in different regions reflects climatic factors such as temperature, precipitation, moisture sources, and transport process. However, the isotopic variation in the region is usually much complicated due to the combined influences of these factors. A good understanding of climatic controls on the isotopic composition of precipitation can contribute to the study on isotopic tracer for climate and hydrology. To investigate the isotopic variation of precipitation and its climatic controls in the middle of the Tibetan Plateau, a monitoring station for stable isotope in precipitation has been established in Nagqu region, central Tibetan Plateau. We obtained 79 daily samples at Nagqu Meteorological Station in 2000. The observed delta O-18 in precipitation showed a distinctly seasonal pattern with higher values in spring and winter and lower values in summer, despite of individually low values in winter due to extremely low temperature. To further understand this pattern, we evaluated the influence of temperature, precipitation, moisture sources, and moisture transport process on precipitation delta O-18. A multiple linear regression model represents quantitatively the dependence of precipitation delta O-18 on precipitation and temperature: delta O-18(ppt) = -0.30P -aEuro parts per thousand 0.11T -aEuro parts per thousand 14.8 (R (2) = 0.13, n = 79, P = 0.005), which indicates delta O-18 values in precipitation are more dependent on precipitation amount than on temperature. In contrast, when the temperature is low enough (< 2A degrees C), delta O-18 values in precipitation are mainly dependent on temperature: delta O-18(ppt) = 0.53T -aEuro parts per thousand 10.2 (R (2) = 0.44, n = 19, P = 0.002). The variation of delta O-18 in precipitation is also closely related to moisture origins and transport trajectories. A model is set up to trace the trajectories for air masses arriving in the observed region, and the results demonstrated that humid marine air masses from the Indian Ocean generally have significantly lower delta O-18 values than dry continental air masses from the north or local re-evaporation. During monsoon precipitation, the distance and depth of moisture transport as well as convective precipitation all lead to the large variability of delta O-18 in precipitation.