Street Canyons in cities are usually poorly ventilated such that pollutant gases emitted from traffic would be trapped in the lower portion of the street canyon. We use a large-eddy simulation (LES) to examine the flow and scalar dispersion inside an idealized street canyon so as to investigate the pollutant distribution and its removal mechanism. The LES domain is configured to include a square cavity topped by a free surface layer. The flow is perpendicular to the street axis and a modeled scalar line source parallel to the street axis is placed at the street level. At a Reynolds number of approximately 12,000, we find a primary vortex at the center core and three secondary vortices at the corners. The primary vortex brings the pollutant towards the leeward wall and upwards to the free surface layer. The mean scalar mixing ratio on the leeward wall is about two to three times greater than that on the windward wall. Analysis of the calculated turbulence parameters shows that the removal of scalar is accomplished by turbulent dispersion at the upstream roof level. Results show that about 97% of the pollutant is retained in the street canyon.