Among the various atmospheric dispersion models, a Gaussian plume model was chosen for Level 3 PSA. The merits of this model are: cost-effectiveness in computing time, unnecessity of complicated atmospheric data, and the comparable output to the detailed computing codes for a flat terrain. In order to complement the simple nature of the Gaussian mode, corrections on the plume rise, the mixing height, the wet and dry deposition, and the radionuclide decay were documented. A stand-alone computer code was developed to simulate the atmospheric dispersion using the Gaussian plume model. Given the wind direction, the wind speed and the atmospheric stability, the code calculates the dispersion coefficient and the corresponding concentration of fission products in the air and on the ground surface. The effect of atmospheric stability change with transport distance is also incorporated into the code. Besides the simulation of atmospheric dispersion, a visualization technique is under development and the part of the results is shown here. When the model for the evaluation of fission product concentration is married with this visualization technique, this system can allow users to see the Level 3 PSA results easily on a PC screen. (Author).