Using observed daily 2-m air temperature data during 1990-2007 and hourly measurements during 2004-2007 recorded at 14 meteorological stations in Beijing, we investigated the annual, seasonal, and diurnal variations of the surface air temperature among four different areas of Beijing: The urban area (UA), suburban area (SA), north mountainous area (NMA), and south mountainous area (SMA). Distinct features were observed in the four areas. For all time scales, due to the urban heat island effect, the maximum surface air temperature was observed in UA, followed by SA, SMA, while the minimum was in NMA. During 1990-2007, surface air temperatures in all the four areas showed a significant warming trend, with the largest warming rate being in UA. The difference in surface air temperature between UA and the other areas was season-dependent. The difference in surface air temperatures between UA and SA and between UA and NMA were largest in winter and smallest in summer, while the largest difference in air temperatures between UA and SMA occurred in the summer, and the smallest was in spring. The minimum daily range of the surface air temperature was observed in UA in each season while the maximum was in SMA. Comparing the surface air temperature of Beijing Observatory with the four areas, the best fit was with UA while significant differences were observed with the other areas at all time scales.