Origin of water vapor, which falls as precipitation over arid/ semiarid area in northeast Asia, is investigated by modeling and observational approaches. The regional climate model evaluates the spatiotemporal variations of precipitation and water vapor budget, which are used to drive the single- layer isotope circulation model. Intraseasonal variations of delta(18) O in precipitation are well simulated during June- July- August of 2003. The delta(18) O in water vapor experiences rapid decreases according to the passage of synoptic- scale disturbances. The rapid decreases of delta(18) O are attributed to two processes. ( 1) The delta(18) O in air mass locally decreases over the western mountains associated with the mountain precipitation; the light vapor is advected to eastern Mongolia by the prevailing westerly wind. ( 2) Convective systems pass by in the vicinity of the observation site, which persistently keep the lower delta(18) O in the air mass. Origin of the water vapor during June-July- August in 2003 is evaluated by the colored moisture analysis in which the tracers are assigned depending on the region where the water vapor finally evaporated. Seasonal mean result indicates that the regions contributing to precipitation in Mongolia are not low- latitude regions but central Asia and western Siberia located to the northwest of Mongolia. Observed multilevel isotopic composition supports the model estimation. The moisture transport along the southwesterly wind of the Asian summer monsoon has difficulty reaching Mongolia as a monthly/ seasonal mean perspective. However, eastern Mongolia and northeast China are situated on the border area between westerly wind moisture transport ( by midlatitude synoptic cyclones) and southerly wind moisture transport ( by Asian summer monsoon).