This paper examines the application of a GIS based planning support system, What If? TM, to evaluate alternative growth policies for sustainable urban growth in a rapidly growing city of Iran. The paper describes the study area, Dorood city, and the What If? Model. It then explains the procedures that were used to obtain the required data and to run the model to Dorood city. The paper concludes by considering the results and implications, which the study has for urban growth management of the city to preserve farmland and accommodate growth in the city.