A new approach based on online coupled liquid chromatography-gas chromatography-mass spectrometry ( LC-GC / MS ) was developed for the rapid determination of 4-( methylnitrosamino )-1-( 3-pyridyl)-1-butanone (NNK) in mainstream cigarette smoke, in which a switching valve was employed to online switching between two-dimensional chromatography. The online LC-GC / MS system used in this study was built by using online gel permeation chromatography-gas chromatography-mass spectrometry except that the micro gel column was replaced by micro alkaline alumina column which was prepared by ourselves before. The NNK in mainstream cigarette smoke was collected by a Cambridge filter pad, then the pad was extracted with dichloromethane, and the extract was quantitatively analyzed by online LC-GC / MS with D4-NNK as an internal standard. Online LC-GC / MS allowed online pretreatment purification, and the sample was subjected to online LC-GC / MS without any prior purification, which reduced human error in analysis process. The injection volume of the present online LC-GC / MS could reach 40 μL, which was 20 times of that of the conventional GC / MS (2. 0 μL of injection volume), and thus significantly improved the sensitivity. Under the optimum conditions, good linearity ( r = 0. 9998) was obtained over the range of 1. 2 - 120 ng / mL. The recoveries at three spiked levels ranged from 93. 9% to 96. 0% , and the limits of detection for qualitative and quantitative detection were 0. 25 ng / mL and 0. 9 ng / mL, respectively. All the results obtained by the present method are comparable to those of standard method recommend by China National Tobacco Company.