Urban heat islands (UHIs),which are urban areas with higher surface or air temperatures than surrounding rural areas,can further enhance the heat stress already exacerbated by global warming. This poses great challenges to human health and sustainable development. China has been burdened with heavy air pollution in recent years,and the effect of haze pollution on UHIs is still far less well understood. This study investigated the spatial variations of daytime,nighttime,and seasonal surface UHI effects in China during 2003-2013,based on MODIS land surface temperature data,and analyzed the correlations between the UHI effect and its impact factors from both biophysical and biochemical perspectives. Our results show that MODIS-derived annual nighttime UHI effect (3.4 K 0.2 K,mean 1 s. e.) is higher than the annual mean daytime UHI effect (2.1 K 0.3 K). The daytime UHI effect is strongest in summer and weakest in winter but contrasts with the seasonal variation characteristics of nighttime UHI effect. During daytime,UHI effects in humid and semi-humid regions are much more obvious than those in semi-arid /arid regions; during nighttime,the UHI effect in semi-humid and semi-arid /arid regions is much stronger than that in humid regions. A Daytime Urban Cool Island effect exists in semi-arid /arid regions during spring,autumn,and winter seasons. Population,vegetation activity,and irrigation are three factors controlling annual mean daytime UHI effect. The nighttime UHI is significantly influenced by latitude,albedo,precipitation,and aerosol concentrations. We provide evidence for a long-held hypothesis that the biogeochemical effect of urban aerosols is an important contributor to the UHI effect. The important role played by rural background environment in calculating the UHI effect is further discussed in terms of surface aerodynamic roughness and the contrast in albedo between urban and rural areas. Mitigation of haze pollution has a co-benefit of reducing the UHI effect and heat stress for urban dwellers. It is also conducive to reducing negative impacts of regional and global climate change.