The　fast　urban　expansion　has　led　to　replacement　of　natural　vegetation-dominated　land　surfaces　by　various　impervious　materials.　This　has　a　significant　impact　on　the　environment　due　to　modification　of　heat　energy　balance.　Timely　understanding　of　spatiotemporal　information　of　impervious　surface　has　become　more　urgent　as　conventional　methods　for　estimating　impervious　surface　are　very　limited.　In　response　to　this　need,　this　paper　proposes　a　new　index,　normalized　difference　impervious　surface　index　(NDISI),　for　estimating　impervious　surface.　The　application　of　the　index　to　the　Landsat　ETM+　image　of　Fuzhou　City　and　the　ASTER　image　of　Xiamen　City　in　China　has　shown　that　the　new　index　can　efficiently　enhance　and　extract　impervious　surfaces　from　satellite　imagery,　and　the　normalized　NDISI　can　represent　the　real　percentage　of　impervious　surface.　The　index　was　further　used　as　an　indicator　to　investigate　the　impact　of　impervious　surface　on　urban　heat　environment　by　examination　of　its　quantitative　relationship　with　land　surface　temperature　(LST),　vegetation,　and　water　using　multivariate　statistical　analysis.　The　result　reveals　that　impervious　surface　has　a　positive　exponential　relationship　with　LST　rather　than　a　simple　linear　one.　This　suggests　that　the　areas　with　high　percent　impervious　surface　will　accelerate　LST　rise　and　urban　heat　island　development.