Urban heat island（UHI） is one of the main factors influencing the weather

climate

and environment over the urban and its surrounding area.In the present study

MODIS land surface temperature retrievals at 1 km resolution produced by the split-window algorithm during the year of 2000—2005 are used to investigate the UHI over Beijing Metropolitan.It was found that the daytime UHI demonstrates distinctive seasonal variation

with the maximum during summer being larger than 10℃ and the minimum during winter being negative

which it is called urban cool island hese.Numerical simulation shows that the urban cool island mainly results from the difference of the surface thermal characteristics between urban and rural area.The seasonal variation of nighttime UHI is much less

being about 5℃ throughout a year.Beijing is surrounded by mountainous area in the west

the north

and the northeast

whose height reach up to 2000m（See Figure 1）.In the east and south of Beijing

the elevation is close to sea level with the complexity of the underlying surface.The different rural baseline not only influences the value of UHI but its seasonal variation.Three typical areas with different land surface types are selected to study the influence on UHI.The results show that the cropland area near Beijing city has uniform vegetation and is suitable to be studied as rural area.The cool island during winter（spring） maybe increase the near surface air stability and aggravate the air pollution over Beijing urban area.