Based on the AVHRR NDVI data and climate data from 1982 to 2006 in the Mongolian Plateau, the temporal and spatial variability of vegetation and its affecting factors were analyzed. The results showed that:(1) The vegetation of Mongolian Plateau had improved as a whole during the recent 25 years with the 0.0006 per year increase rate of annual NDVI. Mongolia had the largest annual increase rate(0.0008 per year), followed by Inner Mongolia(0.0004 per year), Ningxia(0.0003 per year) and Gansu(0.0001 per year) Province of China.(2) The trend of vegetation had obvious differences in spatial and seasonal characteristics. The vegetation in Northwest Mongolia and the Xilinguole of Inter Mongolia had significantly improved in recent 25 years, while the Hulunbeir of Inner Mongolia showed degrading trend. The vegetation had significantly improved in spring and autumn, while degraded in summer.(3) The change trend of vegetation was affected by temperature and precipitation to some extent. Rising temperature extended the length of growing season, which promoted the vegetation growth in spring and autumn. Rising temperature together with reducing precipitation resulted in vegetation degradation in summer.(4) The increased area and yields of grassland and cropland, directly or indirectly, led to NDVI increased, and the other human activities, for example, increased population, changed land cover/land use, returning farmland to forest, overgrazing, exploitation of mineral resources, may also have impacts on the vegetation changes.