Abstract: The organic molecular tracers such as branched alkane and alkylcyclohexane in PM2.5 were determined using direct thermal desorption-gas chromatography-mass spectrometry （GC/MS）. In order to improve selectivity and sensitivity of method, various parameters such as desorption temperature, desorption time, cooling trap packing, and cooling temperature were studied. Results showed that the correlation coefficients of organic molecular tracers in the range of 0.25-50 ng and 0.025-0.5 ng are all above 0.990. The recoveries of the target compounds in blank quartz filters and actual particulate matter samples are 86.2%-110% and 97.1%-113%, respectively. Besides, the limits of detection (LODs) of the target compounds are 0.004-0.020 ng/m³ with the sampling volume of 24 m³, which is much lower compared with accelerated solvent extraction method, because of no solvent extraciton and less target compounds loss. The actual samples of PM2.5 collected in heating season and non heating season in Beijing were determined using this method. The results showed that the target compounds are all detected, and the total concentrations of branched alkane and alkylcyclohexane in heating season are significantly higher than that in the non heating season. Besides, the concentration of branched alkane is two orders of magnitude higher than alkylcyclohexane both in heating season and non heating season. This method is simple and fast, and has great application value in the rapid detection of non polar compounds in particulate matter without complicated pretreatment and organic solvent.