The deserts are suitable areas for large solar power plants, because of abundant sunshine and enormous land. Both are important factors for solar power plants. However, not all desert areas are suitable for installation, for example sand dunes, water, and cliffs. But there is one kind of desert area which has flat and hard ground. It is gravel desert which can be a suitable location. Therefore, the authors proposed a method to identify flat desert areas for large solar power plants using satellite images. It consists of three criteria; (1) Mountainous classification to find cliffs and sand dunes, (2) Ground cover classification by maximum likelihood estimation, and (3) Vegetation classification to find lower vegetation area. With the criterion, a land suitability index map; a detailed map of flat areas, of the Sahara desert had been computed. In this paper, the criterion has been refined in order to evaluate higher vegetation regions which are located at the fringes of desert. The revised criterion was applied to the Sahara desert, and the land suitability index map has been computed.