The　quantitative　relationship　between　the　Normalised　difference　Vegetation　Index　(NDVI)　data　of　Landsat　and　ASTER　sensors　has　not　yet　been　examined　though　they　have　been　frequently　used　in　medium-scale　vegetation　observations.　To　meet　this　need,　this　paper　investigated　the　relationship　of　the　NDVI　measurements　between　Landsat-7　ETM+　and　ASTER.　The　study　examined　how　well　ASTER　sensor　vegetation　observations　replicate　ETM+　vegetation　observations,　and　more　importantly,　the　differences　between　the　two　sensors　in　the　NDVI-based　vegetation　observations.　Three　date-coincident　image　pairs　of　the　two　sensors　were　used　in　this　comparison　study.　The　digital　numbers　of　the　image　pairs　were　converted　to　at-satellite　reflectance.　The　regression　analysis　of　the　scatterplots　of　ETM+　NDVI　versus　ASTER　NDVI　was　carried　out　to　reveal　their　quantitative　relationship.　The　model　for　the　conversion　of　NDVI　data　between　the　two　sensors　was　also　obtained　by　the　regression.　The　results　show　that　there　are　differences　between　the　NDVI　data　of　the　two　sensors　despite　a　very　strong　positive　linear　relationship　between　them.　The　ASTER　sensor　generally　produces　lower　NDVI　measurements　than　ETM+.