Since the heat island researches in Taiwan have been on four metropolitans, the heat island effects on medium and small cities have not been completely and exhaustively studied. Hence, in this research, four medium and small cities – Hsin-chu、Chia-yi、Hsin-ying、Ma-tou in Taiwan are studied in order to know the characteristics of heat island phenomenon in medium and small cities, such as its changes with time and space and its temperature distribution. Moreover, with the establishment of the population and land use GIS, this research, in the fixed quantity, focuses on how the density of population and the types of land influence the urban heat island effects in the medium and small cities in Taiwan.