This study investigated environmental efficiency considering heterogeneity for 192 thermal power plants in the Yangtze River Delta of China from 2009 to 2011. We evaluated environmental efficiency using metafrontier directional slacks-based measure. We also testify whether heterogeneity for environmental technology increased or decreased among different provinces. We proposed a regional, time, and coal intensity effects hypothesis to explore the determinants of environmental efficiency through bootstrapped truncated regression. We found that heterogeneity for environmental technology widened from 2010 to 2011. Shanghai had the highest environmental efficiency. Environmental efficiency increased from 2009 to 2010, and declined in 2011. Coal consumption intensity negatively impacted environmental efficiency. It is critical that we should decrease the rate of coal use, and expand technology spillover of production technology and environmental technology among different provinces and power plants.