The　present　paper　investigates　the　quantitative　relationship　between　the　NDVI　and　SAVI　vegetation　indices　of　Landsat　and　ASTER　sensors　based　on　three　tandem　image　pairs.　The　study　examines　how　well　ASTER　sensor　vegetation　observations　replicate　ETM+　vegetation　observations,　and　more　importantly,　the　difference　in　the　vegetation　observations　between　the　two　sensors.　The　DN　values　of　the　three　image　pairs　were　first　converted　to　at-sensor　reflectance　to　reduce　radiometric　differences　between　two　sensors,　images.　The　NDVI　and　SAVI　vegetation　indices　of　the　two　sensors　were　then　calculated　using　the　converted　reflectance.　The　quantitative　relationship　was　revealed　through　regression　analysis　on　the　scatter　plots　of　the　vegetation　index　values　of　the　two　sensors.　The　models　for　the　conversion　between　the　two　sensors,　vegetation　indices　were　also　obtained　from　the　regression.　The　results　show　that　the　difference　does　exist　between　the　two　sensors,　vegetation　indices　though　they　have　a　very　strong　positive　linear　relationship.　The　study　found　that　the　red　and　near　infrared　measurements　differ　between　the　two　sensors,　with　ASTER　generally　producing　higher　reflectance　in　the　red　band　and　lower　reflectance　in　the　near　infrared　band　than　the　ETM+　sensor.　This　results　in　the　ASTER　sensor　producing　lower　spectral　vegetation　index　measurements,　for　the　same　target,　than　ETM+.　The　relative　spectral　response　function　differences　in　the　red　and　near　infrared　bands　between　the　two　sensors　are　believed　to　be　the　main　factor　contributing　to　their　differences　in　vegetation　index　measurements,　because　the　red　and　near　infrared　relative　spectral　response　features　of　the　ASTER　sensor　overlap　the　vegetation　＂red　edge＂　spectral　region.　The　obtained　conversion　models　have　high　accuracy　with　a　RMSE　less　than　0.　04　for　both　sensors＇　inter-conversion　between　corresponding　vegetation　indices.