In this study, individual precipitation samples, collected over 2 years at stations in different climatic regions of west China (Tibetan Plateau region, Tianshan region, and Altay) were analyzed for the stable isotopes of precipitation to improve our understanding of how vapor transport impacts the modern stable isotopic distribution. Our results identify regional patterns in both δ18O and deuterium excess (D excess, defined as δD – 8δ18O), and in particular we have identified the northward maximum extent of the southwest monsoon over the Tibetan Plateau. This demarcation is also the boundary for the fractionation effect of temperature on stable isotopes in precipitation. The patterns we have identified are as follows: (1) In the southern Tibetan Plateau, along the southern slope of the Himalayas, our results show a distinct seasonality for both δ18O and D excess as a result of the shift of summer monsoon moisture and winter westerly moisture transport. The signals of δ18O in the western Tibetan Plateau reveal that the region receives southwest monsoonal moisture. In the east of the plateau, stable isotopic variation shows alternation between monsoon intrusion and recycling of northern moisture. (2) In contrast, in Tianshan there is an apparent “temperature effect” in δ18O, with enriched values occurring in summer and depleted values occurring in winter. Seasonal D excess values, opposite to those observed in the southern Tibetan Plateau, are controlled by differing seasonal evaporation conditions. (3) In Altay, the most northern mountain region, the seasonal δ18O shows the same variation with that in Tianshan region. However, D excess shows no apparent seasonal variation.