The concentrations of polycyclic aromatic hydrocarbons（PAHs）in different-sized airborne particles(from PM0.25 to PM2.5) were measured in a junior middle school in Xi’an from 8 March 2012 to 22 March 2012.The average PAHs concentration was 82.57 ng·m-3 and the dominant species were Fluorene (FLO), Phenanthrene(PHE), Chrysene (CHR), Benzo[b]fluoranthene (BbF) and Benzo[e]pyrene (BeP). The average outdoor PAHslevel was higher than the indoor level. BaP, the most carcinogenic PAH, was at an average level of 3.24 ng·m-3.PAHs concentration in PM0.25 was much higher than that in larger size particles. Due to its respirable size, itcaused greater health impacts on human, especially for the teenagers in this study. The average BEQ (BaPEquivalent Concentration) was 9.30 and was higher in outdoor air. BaP, followed by BbF and DahA, contributedthe most to BEQ among all measured PAHs, and these three PAHs accounted for 76.37％ of the total BEQ.Combustion is the main source of PAHs in ambient air. The health risk assessment of PAHs for the students wasevaluated.