The urban heat island effect in Lanzhou is analyzed using the average temperature and daily maximum/minimum temperature data from 1956 to 2005. The tendencies and contribution rates of heat island intensity are calculated using the temperature tendencies of the three temperatures of Lanzhou and the suburbs. Results show that the differences of the three temperatures between Lanzhou and the suburbs all exhibit increasing trends. The tendencies of average temperature，maximum temperature and minimum temperature are 0.371 ℃/10a, 0.169 ℃/10a and 0.654 ℃/10a， respectively, with minimum temperature increasing most obviously. In recent 50 years，the temperatures increased mainly in the later 25 years (1981 to 2005), and the tendencies of average temperature，maximum temperature and minimum temperature of Lanzhou are 0.789 ℃/10a， 0.997 ℃/10a, and 0.625 ℃/10a，while those of the suburbs are 0.395 ℃/10a,0.188 ℃/10a, and 0.674 ℃/10a, respectively. The heat island contribution rates of three temperatures are 87.0%，49.6%, and 100%,respectively. In winter, the tendencies of average temperature and minimum temperature are the greatest in both urban and suburb areas，but the heat island contribution rates of the temperatures are not the largest，which may be mainly related with the severe air pollution in winter in Lanzhou，because it can hold back the urban heat island effect in some extent. In the later 25 years， the urban heat island effect of Lanzhou is becoming stronger, but the contribution rates of average temperature and maximum temperature are declining.