This paper analyzes air temperature change tendency, seasonal change characteristics and yearly change characteristics in Nanjing city since 1951 by the meteorological data, such as daily average temperature, daily lowest temperature and daily highest temperature. To know the intensity change and space scope of urban heat island, the daily average temperature in Nanjings and Liuhe's weather station and two-granule TM images in 1985 and 2000 are used respectively. The results show that daily average temperature, daily lowest temperature and daily highest temperature all tend to rise during 1951—2000 in Nanjing city. Temperature change is different for different season. Daily average temperature and daily highest temperature tend to descend in summer but rise in spring, fall and winter. Daily lowest temperature shows the rising tendency in four seasons. In different phase during 1951—2000, temperature rise is different. The rising extent of temperature is the maximal in 1990s.The scope of urban heat island in Nanjing building area has added 10788 km2 since 1985. There were two heat islands in 1985 while the number rose to 3 in 2000.