Abstract: A series of tracer experiments in city street canyons under light wind was conducted to investigate the atmospheric diffusion of vehicle-generated pollutants. When the roof-level wind speed u was near or larger than 1 m/s, standing primary vortex formed in the street canyon, and as u was less than 0.6 m/s, the primary vortex vanished. The ratio of mean tracer concentrations on leeward side to that on windward side was about 8 for street canyons where the space between buildings was small. The tracer concentrations were near constant along vertical direction of buildings faced to the canyon, but it seems higher in the middle of the building heights than otherwheres, because the tracer plume may be resisted and raised by pine trees beside the lane. It was also found that the tracer concentration decayed with distance in a larger negative exponential order for the upwind direction than that for the downwind direction. According to the experiments mentioned above a practical diffusion model was developed for predicting concentration of inert and vehicle-generated pollutants in the city street canyon. In the model the turbulent diffusion was divided into two kinds of motion: the primary vortex and the microscale turbulence