Abstract: The primary purpose of this study is to estimate the boundary between vegetated and non-vegetated areas and to assess the condition of desertification in western China located in arid and semiarid regions. Vegetation is very sparse in desert and therefore onset of green-up in the desert was undetectable with AVHRR NDVI data. The occurrence of onset of green-up, as determined with time series NDVI data, is used to identify desert and non-desert areas. The coefficient of variation (CoV) of the monthly NDVI (maximum-value composite) is used as a parameter to characterize the changes of vegetation in this work.