Abstract: By using the daily total precipitable water amount (TPW) from the ECMWF reanalysis (ERA40) in 1958-2001, the north edge of the Asian sub-tropical summer monsoon was defined by a normalized precipitable water index (NPWI), and its climatology and interannual/interdecadal variability were examined. Results show that two monsoon systems (the Indian monsoon system and the East Asian monsoon system) can be identified in terms of the north edge of Asian summer monsoon defined by the NPWI. The mean north edge goes eastwards along the south side of the Tibetan Plateau in the area west of 100°E, and then turns northeastwards along the northeast side of the Tibetan Plateau via the east of Northwest China, the north of North China, the west of Northeast China to the northeast Asia. The north edge of the East Asian monsoon system has distinct interannual variability in the area east of 100°E, while that of the Indian monsoon does not show obvious interannual variation in the area west of 100°E.