Increased emissions and reduced natural ventilation cause build of high pollution levels in urban street canyons. Nutural ventilation in urban streets canyons is restricted because the bulk of flow does not enter inside and pollutions are trapped in the lower region.In this project, the wind flow and pollutant dispersion phenomena in different street canyons for different aspect ratios (w/h) and height ratios (h2/h1) and different velocities are studied and in fluent software imulated.With rapid development in computer hardware and numerical algorithms, computational fluid dynamics (CFD) techniques are widely utilized to study the wind field and pollutant traort in urban street canyons. The recent advancements and achievements in street canyon pollution research using CFD modelling approaches are reviewed in this project. The k-epsilon turbulence closure schemes are the most commonly used Reynolds-averaged Navier–Stokes (RANS) models in street-canyon flow research, including the studies on the effects of street-canyon aspect ratio, building configuration, ambient wind direction, inflow turbulence intensities. Turbulence models of RNG K-epsilon with species traort without reaction was developed to investigate the flow field and pollutant dispersion inside street canyons. The domain of numerical solution is discretised by gambit software. Aspect ratio of varied from 1 to 4 were employed in the study. These street-canyon aspect ratios covered a range of basic flow regimes including skimming flow (W/H=1 ), wake interference flow (W/H=2/5), and isolated roughness flow (W/H=4) that was mentioned by oke (1988).A pollutant line source, located in the middle of the street, was used to simulate vehicular emissions. Power of law wa implemented for velocity inlet profil.In low velocities, results presented agree with the generally obtained threshold values for different flow regimes mentioned by oke (1988). But increase in velocity has smeared the flow regimes boundry. In higher velocities, pollutant dispersion is mainly promoted by better mixing inside the canyons. Results is showed that the flow regime and pollution dispersion pattern is depend on both the canyon geometry and velocity inlet. the objective this work is to demonstrate the variou flow regimes in different canyon geometries and velocities inlet.The simulation results were compared with experiment of results and favorable agreement was found. Keywords: Urban street canyon; Pollutant dispersion; Canyon aspect ratio K-epsilon turbulence model; species traort model