This study investigates the effects of wind direction on the airflow and pollutant dispersion inside a long street canyon using computational fluid dynamics (CFD). A 3D CFD model for the flow and dispersion in a canyon is first developed using the FLUENT code, and then validated against the wind tunnel experiments. Then, the airflow and traffic pollutant dispersion in an isolated canyon with street-length-to-building-height ratio of 10 are simulated numerically considering seven wind directions (α = 00, 150, 300, 450, 600, 750 and 900, where α is the angle between the approaching flow and street axis). The results demonstrate that: the mean (ACH) and turbulent (ACH′) air exchange rates (ACHs) for the canyon are close under α = 00, 150, 300, 450 and 600, whereas the magnitude of is significantly greater than that of under α = 750 and 900; the magnitude of ACH reaches maximum at α = 300 and minimum at α = 900. The computed velocity and concentration fields reveal clearly the variations of in-canyon flow pattern and pollutant distributions on canyon walls and in canyon footpaths with the wind directions. The maximum, minimum and average concentration levels on canyon walls and in canyon footpaths are evaluated under each of the seven wind directions and it is determined that: on the leeward-oriented wall, the wall-averaged concentration increases greatly with α, and the wall-maximum concentration reaches the highest at α = 750 and lowest at α = 00; on the windward-oriented wall both the wall-averaged and wall-maximum concentrations reach the highest at α = 00; at the human respiration height, the highest concentration in the footpath next to the leeward-oriented wall is found at α = 750 while the highest concentration in the footpath close to the windward-oriented wall occurs at α = 00.