Recently, several phenomena associated with abnormal climate changes on the earth have been receiving attention. The increasing use of fossil fuels and other energy sources and the intensification of energy use in urban environments have given rise to the so called heat island effect that occurs in cities. In this research, we measured and analyzed this heat island effect in Cheon-An city, a medium sized city whose population and urbanization has increased rapidly in the past ten years. Temperature and humidity were measured by the continuous measurement method. The locations of the field measurements were the main commercial area and two residential areas within Cheon-an City Instantaneous measurement was measured 4 times (before sunrise, during the morning commute hours, during the warmest time of the day, and during the evening commute hours) during one day in fall (October 16, 2007) and during one day in winter (February 12, 2008). Temperature, humidity and air velocity were measured. Also, surface temperature was measured in order to analyze the different effects caused by surface materials.