Stable isotopes in precipitation have been extensively evaluated across the Tibetan Plateau. However, the influence of distinct water vapor transport pathways on precipitation isotope ratios during the active and break phases of the Indian monsoon remains poorly understood. Temporal and spatial variations in these isotopes are documented over the Tibetan Plateau and Nepal based on the 1998 GEWEX-GAME/Tibet field campaign. According to these observations, the isotopic composition of precipitation over the plateau was strongly modulated by differences in water vapor transport mechanisms. The transport routes were classified into Indian monsoon and westerlies by employing backward trajectory analysis. The westerlies delivered precipitation with elevated δ¹⁸O and d-excess values (d-excess = δD − 8δ¹⁸O) to the central Tibetan Plateau, consistent with the previous observations. In contrast, the Indian monsoon brought precipitation with distinct isotopic characteristics depending on the monsoon phase. During the active phase, the reduced rainfall south of the Himalayas limited the rainout of heavy isotopes, allowing water vapor with higher δ¹⁸O than the break phase to reach the Tibetan Plateau. During the break phase, the enhanced orographic rainfall along the windward slopes of the Himalayas caused a progressive decrease in δ¹⁸O values toward the north. These findings indicate that the accurate interpretation of stable isotope data in precipitation over the Tibetan Plateau requires consideration of both the active and break phases of the Indian monsoon.