This study was conducted to improve the analysis method used for N-nitrosamines and to investigate the occurrences of N-nitrosamines in tributaries of the Han-river, intake stations, water treatment plants and tap water used within the city of Seoul. The samples were pretreated through a solid phase extraction and analyzed using a gas chromatography tandem mass spectrometer (GC-MS/MS). The GC-MS/MS in CI mode was compared with the GC-MS/MS in EI mode by the method detection limits (MDLs). MDLs by GC-CI/MS/MS and GC-EI/MS/MS were 0.2~1.1 ng/L and 0.2~1.4 ng/L, respectively. Samples were collected from ten tributaries of the Han-river (T1~T10), six intake stations (I1~I6), six water treatment plants (P1~P6) and 25 taps in Seoul city. The maximum levels of N-nitrosodimethylamine (NDMA) were 0.