Twelve polycyclic aromatic hydrocarbons were simultaneously measured in indoor and outdoor air of eight homes in Hangzhou, China in both summer and autumn in 1999. It was observed that the sum of PAHs concentrations in indoor air were ranged from 1.418 to 20.466 íg/m3 in summer and from 3.897 to 29.852 íg/m3 in autumn; the corresponding concentrations in outdoor air were between 1.380 and 20.468 íg/m3 in the summer and between 2.721 and 30.678 íg/ m3 in autumn. The PAHs concentrations in indoor air generally exceeded that in the corresponding outdoor air. It was indicated that the two-, three-, and four-ring PAHs were predominantly in vapor phase, while the fivering PAHs were primarily associated with the particulate phase. The fraction of PAHs in vapor phase will increase with the increase of temperature. Among the 12 PAHs, naphthalene was the most abundant PAHs found in indoor and outdoor air. Both in summer and autumn, it contributed more than 60% to the sum of PAHs. Because of the different functions and ventilation conditions, the concentrations of PAHs in the rooms were bedroom> kitchen>living room>balcony. By the contrast of BaP concentrations in smoker and nonsmoker’s homes, we know that smoking in indoors could contribute 67% of BaP to the homes.