Abstract: Using Landsat-7 ETM+ digital spectral data investigations have been carried out in northwestern part of Yunnan province (Heqing area) to generate the geological mapping interpretation. Landsat Enhanced Thematic Mapper (ETM) of (945�1232) pixels subscene acquired on 21 March 2000 covering the northwestern part of Yunnan province has been digitally processed using ER Mapper software. The aim of this paper is to improve geological interpretation map for the study area. To meet the purposes of this research, different image processing methods have been evaluated in context of geological mapping; involving color composite image, Principal Component Analysis (PCA) and IHS decorrelation processing, appear to provide the maximum geological information. Color composites techniques were used the best suited approach is given by color combination of bands 531 in (RGB) provided the best results based on the original data. In principle, comparable results can be generated by a combination of the Principal Components (PCs) 1, 4 and 7 (blue, green and red respectively) in this succession. Decorrelated and filtered color composite of ETM+ bands 1, 4 and 7 have proved to be more successful for delineation the regional geological features. IHS-transformed images, with substantially saturated color, appear generally superior in discriminating the various rock units when three least correlated bands are used. The study testifies that using Landsat-7 ETM+ data PC and IHS decorrelation-stretching methods yield the best results for geological mapping in arid regions, by preserving morphologic and spectral information and when combined, they can be very helpful for improvements in already mapped areas.