One of the most obvious climatic manifestations of urbanization in Korea is a trend towards higher air temperature. The trends of long-term annual temperature generally well describe the warming of urban areas. The increase of air temperature in urban area has been observed to the present since the meteorological observations in Korea began. The objective of this study is to explore the actual increase and the regional long-term trends of air temperature attributed to urbanization in the Korean Peninsula. Therefore, temperatures of the selected urban areas were compared with that of the surrounding rural areas, with the results varying by the application of the estimates of each region. The second objective is to separate the long-term trend of surface air temperature of global warming from urbanization and to find the actual temperature increase from urbanization in Korean peninsula. For the data analysis, daily air temperatures observed by the Korea Meteorological Administration (KMA) during between from 1961 and 2005 were used at five rural sites and cities. The re-analyzed surface air temperatures by the National Centers for Environmental Prediction (NCEP) was also carried out to compare the result from the observed air temperature in the Korean climate domain. In this study, the urban areas in Korea showed high increase rate of air temperature with $0.4^{circ}C$ per decade during past 50 year period, while rural sites as Chupungryung with the $0.2^{circ}C$ decadal increase rate. The analyses reflect that the urban area shows the high rate of temperature increase with $1.39^{circ}C$ of regression value at the urban area, Seoul, and $0.43^{circ}C$ at the rural site, Chupungnyeong during the period of 30 years. The temperature increas due to the urbanization only showed the increase range between $0.44^{circ}C$ and $0.86^{circ}C$, and the observed decrease in diurnal temperature range at five urban areas during the 30 years period.