Air pollution is one of the most important problems in megacities. To provide comprehensive reduction plan for air pollution and pollutants emission prevention, first it must be estimated the amount and importance of major pollutants emission from various sources. The aims of this study were estimation the air pollutants emission from mobile sources and spatial modelling for each pollutant in Karaj. International vehicle emission model (IVE) was used to obtain air pollutants emission factors based on vehicle type. Finally, the annual pollutants emissions of vehicle fleet were calculated and spatial distribution of pollutants emissions based on traffic data and road networks were mapped in GIS environment. The results showed that the maximum annual emission of air pollutants including carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx), volatile organic compounds (VOCs), particulate matters (PM) and carbon dioxide (CO2) were estimated about 1688.84, 185.61, 6.26, 31.16, 120.54 and 33737.04 t y-1, respectively. The highways and the heavy vehicles were the major source for PM and NOx. In addition, the arterial roads and light vehicles were the main source for CO and VOCs. Despite the highest SOx emission factor was for heavy vehicles, the total SOx emission from light vehicles was higher than heavy vehicles because of their numbers. Also, the highest emissions amount of pollutants in municipal districts of Karaj were in 3, 4 and 6 and the lowest amount were in 1, 8 and 9.