The urban heat island (UHI) effect is one of the many typical ecological problems caused by rapid urbanization both in China and all over the world. Understanding the dynamics of the UHI effect in urban areas is essential to the improvement of human settlement and better urban planning. In this study, a region enclosed by the six-ring road of Beijing was adopted as the sample area, due to the rapid urbanization processes it has undergone over the past 20 years. Based on the land surface temperature (LST) retrieval methods from Landsat-5 TM in summers of 1991, 1993, 1996, 1999, 2004, 2007, 2009, and 2011, indicators of UHI intensity (HII) and capacity index of UHI (HCI) were established. Furthermore, the global spatial autocorrelation index and a gravity center model were used to describe the temporal and spatial dynamics of the UHI effect in the study area quantitatively. The results show that the average HII fluctuated from 5.73℃ to 9.27℃ from 1991 to 2011, with an increasing rate of 1.31℃/10a, indicating the worsening trend of the thermal environmental conditions in the study area over the past 20 years. With regard to the spatial patterning, changes occurring in north-south dimension were significantly more evident than those in the east-west dimension. The overall pattern tended to be fragmented and this can be demonstrated by the decrease in Moran's I index from 0.63 in 1991 to 0.16 in 2011. The gravity center model showed that the UHI effect spread outwards with the urban planning activities in circle ring, from weak to intensive expansion. The location of the gravity center was initially in the zone of the second-ring, the center of case area, and then transferred successively from east to north and south, and finally returning back to the center. The enclosed area of the second-ring showed evident improvement of thermal environmental conditions, due to the changes over time of the region's function.