The Thai's policy in improving of fuel quality to the changing of emissions and concentrations of air pollutants in Bangkok is evaluated in this study. Benzene content in gasoline had been decreased from 3.5% (EURO II standard) to less than 1.0% (EURO IV standard) since the year 2012. The IVE (International Vehicle Emission) model was applied to develop emission factor of benzene taking into consideration actual fleets and characteristics of vehicles in Bangkok. These vehicles consisted of

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... nger car, motorcycle, van and pick up, taxi, bus and truck. Then, emission inventory of benzene emitted from mobile sources was calculated. It was found that about 83% of total emissions of benzene were contributed from motorcycle (43%) and passenger car (40%). Emission amounts were reduced from 1.32 x 10 3 ton/year to 0.57 x 10 3 ton/year as resulted from changing of fuel quality from EURO II to EURO IV standard. AERMOD dispersion model was used to simulate ambient ground level concentration of benzene prior and after implementing this policy. Results revealed significantly reduction of benzene concentrations in Bangkok's environment. Spatial distribution of benzene pollution map illustrated that areas having annual concentration higher than 1.7 g/m 3 (the Thai's air quality standard for benzene) were still only found in traffic-congested zone of the central business district in the metropolitan area.