Abstract: The urban heat island is probably the most serious source of systematic error so far identified in land surface climatological measurements (IPPC, 1990). the development of temperature differences between cities and their surrounding rural regions has been well documented (Landsberg, H. E., 1981). the heat storage differences between the materials that are associated with urban and rural surfaces are responsible for the development of observed air temperature differences. the degree of urbanization effects on the annual mean temperature of Beijing has been probed into in this paper. Beijing is the capital of China. Its administrative area is 16 808km² including suburban districts and counties of outer suburb. There was no any modern industry in Beijing before 1949. the consume of energy is about 3×10⁷ tons of standard coal in 1992. The population (1.1×10⁷) of Beijing in 1992 is more than 2.5 times as many as that in 1949. the population density of Beijing urban districts is 27 629 persons/km² in 1993 and the highest density is 54 008 persons/km² in the same period. This paper edscribesa detailed study of trends of the time series (1960-1993) of surface air temperature in Beijing area and the effects of urban heat island. Time series of monthly mean surface temperature of 7 meteorological stations in Beijing area were examined using moving average, and then principal component analysis and trend analysis were used to probe into urbanization effects on the annual mean temperature. The results show that: (1) The first principal component expresses the variation of the annual mean temperature and degree of urbanization effects. The tendencies of the first principal component are negative except beijing and tongxian. The estimated degrees of urbanization effects are about 0.21℃ and 0.19℃/33 years for Beijing observatory and Tongxian station respectively from 1960 to 1993. In the downtown of Beijing, the value of increased temperature by urbanization effects should be more stronger than that in the both stations. The second principal component expresses the quasi cycle of 14 years and the trend of annual mean temperature change in Beijing area. (2) The first eigenvectors are positive, and the amplitudes of variation for Beijing and Tongxian stations are greater than other stations in winter. This, maybe, is caused by stronger urban heat island in winter due to more energy consumed for building warming. The second eigenvectors are the same for all stations in Beijing area. The tendency of warmer winter and cooler summer in Beijing area are showed clearly. (3) It is obvious that temperature records of meteorological station in a city or suburbs are influenced by urbanization or urban heat island. But it is difficult to determine the degree of urbanization effect on temperature records exactly. The author thinks that the method of this paper should be further investigated. (4) Many long term monitoring stations are located in and around the growing urban area, such as Beijing and Tongxian Stations, which can lead to increases of temperature unrelated to global forcing such as the green house effect. So the element of urbanization effects should be paid attention to when we study on regional and global climated change.

Key words: urbanization effect, urban heat island, temperature, principal component analysis, trend analysis