Land surface albedo,defined as the fraction of incident solar radiation reflected in all directions by the land surface,is one of the most important parameters controlling the climate change.Albedo directly determines the amount of solar energy absorbed by the ground,and hence,the amount of energy is available for heating the ground and lower atmosphere and evaporating water.The study on albedo changes through space and time is crucial to understand the global radiation balance and its influence on climate and vegetation dynamics.In this study,the intra-daily,daily,monthly and seasonal variation of land surface albedo in the North Tibetan Plateau is analyzed using the ground observation data and then compared with MODIS derived albedo using narrowband to broadband conversion algorithms of land surface albedo developed by Liang.The results show that the intra-daily changes of albedo in the North Tibetan Plateau is obvious, which indicated that it is high at mooring and afternoon with a larger range of changes,and low at noontime with a smaller range of changes,which is characterized byUtype.There is a statistically significant negative relationship between the observing albedo and the Solar Elevation Angle(SEA)with a correlation coefficient of-0.91,which implys SEA is main factor affecting seasonal changes of albedo.The albedo is deceasing with increase in SEA from morning to noontime,and the lowest value of albedo occurs in 14:00to 15:00,then,albedo is increasing with decrease in SEA from noontime to afternoon.The highest albedo occurs at 8:00of the morning in summer and at 18:00of afternoon in other seasons.At seasonally,the highest albedo is in winter with 0.28,followed by spring and fall with 0.23,and the lowest averaged albedo is 0.19in summer.The albeo derived from MODIS is very consistent with from ground observations at around 12:00MODIS crossing time.However,at about 13:00MODIS crossing time,MODIS derived albedo is systematically lower than observation data with lower than 14.29%on average.Compared to the ground observation data,the inter-daily albedo from MODIS is more fluctuant and less smooth.