Using the monthly mean data of ISCCP D2 and NCEP/NCAR from July 1983 to September 2001,the distribution of 15 different cloud types in three climatic subregions in Northwestern China were analyzed.Also the trend of cloud amount and their possible formation process were discussed.The results show that the optical thickness and water path of liquid stratus,ice stratus,liquid nimbostratus,ice nimbostratus and deep convective clouds are the highest.Liquid stratus mainly occurs over Tianshan Mountains,Northern Xinjiang and Southern Shanxi and ice stratus mostly occurs over Northern Xinjiang.Liquid nimbostratus,ice nimbostratus and deep convective cloud,and also liquid altostratus,ice altostratus and cirrostratus,with values of optical thickness and water path at the second highest level,have a similar distribution.Their high value areas are over Tianshan Mountains,Kunlun Mountains,Qilian Mountains,Southern Shanxi and Southern Gansu.Therefore,those regions have advantage for artificially enhanced precipitation.During the nearly 20 year periods,the trend of the high-level cloud amount significantly decreased while low-middle-level cloud amounts increased in all of the three climatic regions.Such a process probably exists between the cloud and earth-atmospheric system in northwestern China that the increase of surface temperature enhances evaporation,thus causing the increase of the low-middle-level cloud amounts and leading to precipitation to increase and the high-level cloud amount to decrease.