Twenty-one polycyclic aromatic hydrocarbons (PAHs) from a four-stroke motorcycle were collected with a micro-orifice uniform deposit impactor and measured in size-segregated particulate samples of 0.056–18 μm aerodynamic diameter. The motorcycle exhausts were passed through a dilution tunnel to simulate reactions between ambient air and exhaust. The PAHs were analyzed using Gas Chromatography/Mass Spectrometry (GC/MS). The size distribution of particulates was unimodal with a peak at 0.1–0.18 μm. Lower molecular weight PAHs predominate in motorcycle exhaust. The emission factors increase as the particle diameter decreases for PAHs with low molecular weights. Two peaks appear at 0.056–0.1 μm and 0.18–0.32 μm for most middle and higher molecular weight PAHs. For PAHs, the cumulative fractions attributable to diameters less than 1.0 and 2.5 μm were 77.0% and 84.3% for the test motorcycle. The mass median diameter of total PAHs was 0.45 ± 0.11 μm. The BaPeq emission factor for 0.056–18 μm aerodynamic diameter is 70.8 ± 38.4 ng/km for the test motorcycle. High molecular weight PAHs have the highest BaPeq emission factor, indicating that higher molecular weight PAHs are more carcinogenic.