The characteristics of Lanzhou urban heat island effect and the major meteorological factors resulting in the seasonal difference and the inter-decade change trend of heat island effect were analyzed by using temperature data from 1958 to 2003 of Lanzhou and two rural observation stations near Lanzhou.The results indicate that urban island heat effect in Lanzhou strengthened increasingly in the recent 40 years,and the intensity is greater in winter;as for daily variation,the heat island effect at 02: 00 is most obvious,but that at 14: 00 was less obvious.The winter inversion layer and the characteristics of the urban underlying surface in summer are the main factors leading to seasonal difference of heat island effect.The urban development resulted in the increasing heat island effect,and the abnormality of some meteorological factors aggravated the heat island effect in Lanzhou.