ABSTRACTTraffic emission contributes significantly to air pollution in Hanoi. This study estimated emissions from passenger fleets of cars, taxis and buses in Hanoi in 2010 using International Vehicle Emission (IVE) model for 14 species of air pollutants and greenhouse gases (GHGs). Surveys were conducted to gather information on fleet technology distribution and driving activities. Results showed that the 2010 annual emission from three fleets for CO, volatile organic compounds (VOC), NOx, SOx and particulate matter (PM) were 39.5, 5.9, 3.8, 0.6 and 0.22 Gg, respectively. Gasoline-fueled taxis and cars had the major shares of CO and VOC, while diesel-fueled buses contributed mainly to PM and black carbon (BC) emissions. If all vehicles of three fleets conformed to Euro3 and Euro4, air pollution emissions would collectively reduce by 85 and 88%, respectively. Concurrently, emissions of climate forcing agents, both GHGs and short-lived climate pollutants, in CO2 eq. would reduce by 28 and 12%, respectively. Incorporation of emission from motorcycles (MC), vans and trucks showed that MC contributed the highest shares in total emission of every species, from 36% for CO2 to above 90% for air toxics. The MC fleet should be prioritized for traffic emission control. Faster Euro technology intrusion in Hanoi would bring in significant co-benefits.