Analyzed results of the atmospheric wind speed and stable isotopic data (δ18O) in summer precipitation at Bangkok, Bombay, New Delhi, Kunming and Lhasa, the IAEA/WMO stations, indicate thatδ18O in monsoon precipitation correlate positively to wind speed and that there exists a monsoonal vapor layer over these monsoon- controlled areas during monsoon seasons. The isotopic exchange happens between monsoon vapors and falling raindrops in the layer, resulting in this correlation betweenδ18O and wind speed. This suggests that wind speed is probably one of key factors affecting theδ18O variation besides air temperature and rainfall in the southwest monsoon domain.