Abstract: In this study, the maximum downwind distance xmax was calculated according to the dispersion parameters and by using four methods the power law function, standard, Klug and Pasquill-Gifford method sat three effective heights of 5, 100 and 250 m, respectively. The maximum ground level concentration of air pollution that based on the Gaussian model can be calculated by using the estimated and actual values of the x_max for different stability classes. The estimated and actual x_max are summarized as 0-492 m in power law and standard method, also the percentage of the error between estimated and actual maximum downwind distance in the range of 0.005-14.2, respectively. Comparison between the results of estimated and actual x_max in case of power law and standard method with the previous work used Brigg method (Ronbanchob2015) is carried out.