Abstract: The Weather Research Forecasting (WRF) numerical model and its coupled single-layer Urban Canopy Model (UCM) are used to simulate the Urban Heat Island (UHI) effect over Lanzhou during 25-28 January 2005. In order to evaluate the impacts of urbanization over Lanzhou, the land use data that inversed from the MODIS 1 km resolution data is applied. A sensitivity experiment without urban underlying surface is designed to investigate the effects of urban underlying surface on urban boundary layer structures. The main results are as follows: The urban underlying surface increases the surface air temperature and decreases the wind speed, especially in the night. The contribution rate of the urban underlying surface to Lanzhou UHI intensity is 44%. The urban surface air keeps the characteristic of neutral stratification at nighttime. The upward movement of UHI circulation promots the mountain breeze circulation, and reaches the height of 600 m. Due to the heating effects of mountain peak, there is an inversion layer existing at the height of 400-600 m above the city at daytime. The UHI circulation restrain the formation and development of the valley breeze during 11:00-15:00 BT. The low albedo characteristics of urban underlying surface and the multiple reflection of buildings lead the net radiation in urban areas to being more than that in non-urban areas. Because of materials waterproofness of buildings, the latent heat flux is far less than sensible heat flux with the heat storage capacity enhanced obviously.