Abstract: To explore the urban heat island (UHI) at medium-sized cities in the Northwest, the difference of temperature in the urban area and suburbs was analyzed with the meteorological data in automatic meteorological stations in Tianshui city from 2004 to 2010. The result showed that the heat island was obvious in urban area of Tianshui, with the strongest heat island appearing in winter and the weakest in summer. The urban area played as a “heat island” during 18:00 to 10:00 which was two-thirds of the day with the biggest UHI intensity of 1.5 ℃, and as a “cool island” during 11:00-17:00 which was one-third of the day with the biggest “cold island” intensity of 0.3 ℃.The surface temperature varied more obvious than air temperature between urban area and suburbs. The surface temperature below 40cm in the urban area was higher than that in suburbs throughout the whole year. The urban area basically played as a “cool island” during snowy days and long-range rainfall days. There was no significant difference of temperature between urban and suburb areas during the strong temperature drop days.