Abstract: Urban Environment has attracted ever more attention at present. Nanjing is a densely-populated city in lower-reaches of the Yangtze River. Knowledge of land surface temperature distribution in Nanjing can help people understand urban climatic condition, especially urban thermal environment, improve the habitat and make scientific layout for urban development. In order to understand the land surface temperature distribution of the city, with the aid of ASTER thermal infrared data and split-window algorithm, its land surface temperature (LST) is retrieved under the support of ENVI software and then land surface temperature field is established. Comparing ASTER LST with ETM⁺ LST, the results are very accordant. Comparison between ASTER LST and actual measurement data in meteorological station results in a difference of 0.9℃, it is within the range of permissible error, indicating that LST from ASTER is reliable. Results show that there is obvious heat island effect on Nanjing city, the land surface temperature in urban is higher than in suburbs, and there are many cold islands in the urban parks with the larger water bodies and thick plants. The land surface temperature of different space has great discrepancies: The maximal temperature is 56 ℃, the minimum temperature is 23 ℃, with a difference of 33 ℃. There is notable difference in surface temperature among different land uses: The surface temperature is the highest in the regions of asphalt concrete surface in the sun; it is higher in sparse vegetation regions than in dense vegetation cover regions; it is higher in sparse human regions than in dense human regions; the minimum temperature is found over the Yangtze River. On the whole, the surface temperature is higher in the south of Yangtze River, Dachang and Qiaobei than other regions, where it is generally more than 40 ℃. LST exceed 45 ℃ in some districts, including Dachang, Nangang, Qiaobei, Zhongyangmen, east bus station, Xinjiekou, Fuzimiao, Zhonghuamen, the west of Qinhuai River and Yuhuatai, showing characteristics of fleck. The regions where LST exceed 48 ℃ concentrate in Zhongyangmen, Xinjiekou, east bus station, and west of Qinhuai River. The temperatureis lower than 35 ℃ in Zijin Mountain, Xuanwu Lake, Qinhuai River and some urban parks with the larger water bodies and thick plants and LST of Yangtze River is under 30 ℃. With the expansion of city, the heat island effect is even more remarkable in the newly urbanized regions, but the water and the highly dense vegetation in urban help mitigate the thermal effect.