Abstract: The Monte-Carlo model is used to predict the ground level concentration distribution (GLCD) of SO2 in mountainous city—Guiyang city. In order to accomplish the numerical simulation of the area source in the city, the concentration count and horizental diffusion in the Mont-Carlo model are studied. The wind fields used as the background or the input parameters are obtained from the objective analysis according to the three dimentional wind observations, and the turbulent parameters are determined by the parameterized formulas which have been adjusted with the local experimental data in Guiyang city. It is shown that the wind profile is not highly coincided with the ordinary features, there is usually an ‘extremum’ between the layers of 50 and 500m, there are the typical features of atmospheric diffusion under the light wind in Guiyang city, the spectral peak values of the velocity components u and v shift to the lower frequencies and the horizental diffusion remarkably increase. Additionally, the modeling verification both with other theories and the experiments have been done. The GLCD of homogeneous turbulence consists with that obtained from K-theory or Gaussian model. For the daily average values of GCLD in winter, the correlation coefficient between prediction and observation is 0.96. The GLCD simulated shows that the air-pollutant transport and diffusion in Guiyang city is mainly depended on the very unstable weather condition or the aeration effect of the ‘extremum’ wind layer.