The temporal and spatial variations of normalized difference vegetation index(NDVI)in Zhangye region during 2000—2008were analyzed using the gravity center of vegetation

linear regression and R/S analysis based on the MODIS—NDVI data.The results show that the gravity center of vegetation moved eastward during 2000—2003

indicating increases of vegetation cover in the east and decreases in the west. The gravity centre of vegetation moved northward during 2006—2008

which revealed the vegetation increases in the north and decreases in the south during this period.Annual variations of the vegetation gravity centre were unstable from January to May and from November to January

but stable from May to November. Vegetation cover increased in basins of Shandan

Minle and Zhangye during recent ten years

which accounts for 75.98%of the whole study area.The main land use types of the study area were dry land and artificial grassland.The artificial vegetation increased faster than natural vegetation.Degraded regions distributed sporadically over Zhangye Basin and Qilian Mountain due to intensified human activities.The Hurst index of the growing season NDVI focused on 0.4~0.6

indicating weak sustainability and/or non-sustainability. However

the indices of each month in the growing seasons differed substantially.The Hurst index showed a transition of vegetation cover from sustainable in May to unsustainable in July

while the sustainability of the vegetation cover increased in August and September.The results also show different land use types in the order of increasing Hurst value as:unused landⅡ