Monitoring and assessing karst rock resulting from desertification is important to local sustainable development in the karst mountain regions of Southwest China. A new derived index, the Normalized Difference Rock Index (NDRI), is proposed to map karst rock. It takes advantage of the unique spectral response of karst rock and other land cover forms. Supervised maximum likelihood classification trials using different input bands (Landsat TM band 3–5, NDVI and NDRI derived from TM imagery) were assessed for accuracy of karst rock mapping in Bijie County, Southwest China. Results were compared with land use maps for 2000, with the best result obtained using a combination of the NDVI and NDRI indices. This paper offers a tool for land cover mapping.