This book provides numerial results of flow structure and pollutant dispersion in a 3D symmetric street canyon.The analyzed parameters are building aspect ratio and Reynolds number. The studied canyons are avenue, regular and deep canyon with aspect ratio of 0.4, 1 and 2 respectively. Transport equation for CO and air is included and is assumed as non-reactive. Analysis on the pollutant concentration shows that low building aspect ratio coupled with high Reynolds number will give the best air ventilation for street canyons subsequently decreased the pollutant concentration level.