Land vegetation plays a major role in the global climate change through carbon cycle

and climate change in turn afˉfects vegetation growth and its photosynthetic activity.In arid and semiarid area

sparse vegetation cover characterizes environˉments.Thus quantitative temporal series analysis of vegetation distribution and its variations enables observing annual trends

helps to find out the reason for environment variability.There are serious environmental problems

such as deforestation

soil erosion

salinization

and desert encroachment in the northwestern China

its natural conditions is very delicate.In this paper

we built the time series of vegetation change by the NDVI（normalized difference vegetation index）covered northwest regions in recent20years

and analyzd the time serial NDVI variability using three methods

which are simple differencing

slope map of NDVI（and biomass）

and principal component analysis（PCA）.The change image demonstrated that the area of vegetation covers decrease in recent20years in northwestern China

especially in the Loess Plateauwhere precipitation decrease and temperature increase.Only little part

such as the Tianshan Moruntain area

vegetation is increased.The character of vegetation change in northwest showed that the ecological environment in this area is becoming more fragile

such as shrinking of lakes

oasis and wetland

area extending of soil erosion

salinization land

and desert because human activities and global warming.The correlation analysis between NDVI with the temperature and precipitation in northwestern China was carried out.There was significant positive correlation between NDVI and precipitation

and temperature was not strong correlation relatively with NDVI in the northwest China.The results showed AVHRR is reliable for regional vegetation change research after comparingwith the change of precipitation and tempoerature.