Natural climatic conditions in urban areas can be changed by human physical environment. Climate as a natural system and city as a human system have close mutual connections. Urban population growth and consequent increase in urban physical expansion may lead to intensification in human activities and disturbances in the environment. This contributes to alterations in solar radiation, balance of energy in urban atmosphere and eventually undesirable changes in the urban climate. In this study, average of Diurnal Temperature Range (DTR) has been applied as an index of climate change. This index indicating the difference between maximum and minimum temperatures has variations with their increasing or decreasing trends. To examine DTR changes in Tehran, we have used minimum and maximum temperatures and cloudiness data of Mehrabad Synoptic Station from 1956 to 2006. The results of the examination have indicated that there are slight changes in urban climate. This is accompanied with changes in urban population growth and consequent urban physical development.