Abstract: For simultaneous determination of ethylene oxide and propylene oxide in mainstream cigarette smoke, a gas chromatography-mass spectrometry (GC/MS) method combined with cold trapping was developed. Mainstream cigarette smoke was generated under ISO smoking condition and passed through a Cambridge filter pad followed by dry ice cooled impingers containing toluene. The levels of ethylene oxide and propylene oxide in the vapour phase of mainstream cigarette smoke were quantified by GC/MS. The samples of 21 domestic and imported cigarettes with different tar deliveries were analyzed using this method. The results showed that:1) The method displayed a good linearity (R²≥0.9997). The limits of detection (LOD) for ethylene oxide and propylene oxide were 0.03 μg/cig and 6.8 ng/cig, respectively. The recoveries of ethylene oxide and propylene oxide ranged from 91.0% to 101.5% with the intra-day and inter-day precisions between 2.4% and 6.0%. 2) The levels of ethylene oxide and propylene oxide in the mainstream smoke of 21 cigarette samples were between 1.4-17.8 μg/cig and 158.8-1153.0 ng/cig, respectively. This method is simple, reliable, sensitive and suitable for the determination of ethylene oxide and propylene oxide in mainstream cigarette smoke.