The flow and diffusion around a building under neutral(Rib=0) and stable stratification(Rib=0.057) conditions were investigated by wind tunnel experiment. The mean flow and turbulent structure around the building were measured by a combination of hot wire and cold wire. The characteristics of pollutant concentration distribution under neutral and stable stratification conditions were investigated by using tracer experiments. The results indicate that when atmospheric stratification is stable, the longitudinal velocity and turbulence intensity are lower than under neutral conditions. Different thermal stratification has different effect on the diffusion around a building. Stable thermal stratification inhibits the diffusion of pollutants in the vertical direction, which causes an increase in concentrations. The maximum normalized concentration appears near the building(x/H=1/3, z/H=1.2). The maximum normalized concentration is 11 for the stable condition and 7 for the neutral condition. As the downwind distance increases, the height of the concentration peak decreases. Compared with neutral thermal stratification, stable thermal stratification gives a lower concentration peak value.