The　successful　launches　of　many　new　advanced　sensors　on　board　remote　sensing　satellites　since　the　latest　1990s　have　brought　a　new　remote　sensing　study　field,　i.e.,　cross　comparison　between　multi-sensor　imagery.　This　paper　focuses　on　the　comparison　between　Landsat-7　ETM+　and　ASTER　sensor　imagery　because　the　cross　comparison　between　the　two　sensors　has　not　yet　been　implemented.　This　comparison　was　based　on　the　Modified　Normalized　Difference　Water　Index　(MNDWI),　derived　from　three　synchronous　image　pairs　of　two　sensors,　and　was　carried　out　using　three　different　data　types,　which　are　digital　number　(DN)-based,　illumination　correction　model　(ICM)-based　and　illumination　and　atmospheric　correction　model　(IACM)-based　data.　The　regression　analysis　for　the　MNDWI　images　of　the　two　sensors　has　revealed　the　interconversion　relationship　between　the　MNDWI　images　of　the　two　sensors,　and　thus　the　cross-sensor　MNDWI　conversion　equations　for　the　two　sensors.　The　analysis　also　indicates　that　the　MNDWI　of　ASTER　imagery　has　a　strong　positive　correlation　with　that　of　ETM+　imagery　with　R2　values　of　greater　than　or　close　to　0.9.　The　accuracy　verification　shows　that　all　the　conversion　equations　have　high　accuracy　with　a　RMSE　ranging　from　0.04　to　0.09,　and　the　IACM-based　equation　has　achieved　the　highest　accuracy　among　the　three　data　types.　Therefore,　this　paper　suggests　the　use　of　IACM-corrected　data　for　the　interconversion　of　MNDWI　between　the　two　sensors.