The spatial and temporal variations of urban heat islands(UHI) over the Yangtze River Delta region have been analyzed by using MODIS(MODerate-resolution Imaging Spectroradiometer) daytime land surface temperature(T_s), MODIS land cover types and DMSP/OLS nighttime light imagery. Results indicate that the UHI effect over the Yangtze River Delta has strong seasonal variations. The UHI effect observed from the daytime T_s is the most significant in spring and summer, weaker in autumn, and the weakest in winter. Since 2001, the summer heat island areas over the Yangtze River Delta region have increased rapidly, among them growth rate of strong heat island area is the largest, leading to significant decreases of the transition region. The area of strong heat island index has been increased dramatically, especially over the Suzhou-Wuxi-Changzhou city clusters, which is connecting the UHI over Shanghai. This expanding UHI is extended along the costal line to Hangzhou and nearby cities, which is forming a giant heat island. Further studies show that the Ts has the largest increase over suburban region where the nightlight has increased most in the past decade. Over the centers of cities, the nightlight has small changes and the T_s has small change as well. There is a good spatial correlation between the T_s and the nightligh. This suggests that the human activity(urbanization) is highly responsible for the local climate change.