Abstract: Organic aerosols were sampled by applying medium-flow PM_2.5 sampler during September 2003 to July 2004 in Beijing. This study built an optimum condition on the basis of solid phase extract(SPE) for the pretreatment of three kinds of compounds: n-alkanes, PAH_s and organic acids. The derivatization reagent for organic acids was 14% BF₃/CH₃OH, and the optimal derivatization time and temperature were 40 ℃ and 45 min, respectively. By using the self-filled SPE-Silica gel column, the above three kinds of compounds were seperated completely, and the recovery ranged from 70%—130%. The PM_2.5 samples were determined quantitativly by using GC/MS, including 25 alkanes, 16 priority controlled PAHs by USEPA as well as 35 organic acids.