The urban heat island (UHI) phenomenon, which is characterized by higher air temperature in densely built areas than that of the surrounding rural area, can be regarded as the most representative and documented manifestation of climate modification. Although the UHI is a well-documented phenomenon, relatively little information in the literature is available about its impact on summer extreme heat events. With global warming forecast to continue into the foreseeable future, extreme heat events are very likely to increase in both frequency and intensity. As UHIs are characterized by increased temperature, they can potentially increase the magnitude and duration of heat waves within cities. Based on daily maximum temperature records from 62 observation stations in Zhejiang Province from the period 1971-2011 and satellite-measured nighttime light imagery from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) during 1992-2010, we analyzed the change of summer extreme heat events and its association with the rapid urbanization process. The results could be concluded as follows: (1) Zhejiang Province has experienced rapid urbanization and a dramatic growth in urban areas in the past two decades, especially after 2000. (2) The summer mean maximum temperature and the 95th percentile of summer daily maximum temperature in most stations of Zhejiang Province have increased, with the most significant increase in the highly urbanized areas including the city belt around Hangzhou bay, Taizhou-Wenzhou and Jinghua-Yiwu city belts. (3) The hot days and hot day degrees, defined by both daily 95th percentile and the threshold of 35℃, show that the UHI effect causes additional hot days and heat stress in urban stations compared to rural stations. The results in this study suggest that the heat island effect should be determined and incorporated in preparing high temperature forecasts.