The　recently　launched　Landsat-9　has　an　important　mission　of　working　together　with　Landsat-8　to　reduce　the　revisit　period　of　Landsat　Earth　observations　to　eight　days.　This　requires　the　data　of　Landsat-9　to　be　highly　consistent　with　that　of　Landsat-8　to　avoid　bias　caused　by　data　inconsistency　when　the　two　satellites　are　simultaneously　used.　Therefore,　this　study　evaluated　the　consistency　of　the　surface　reflectance　(SR)　and　land　surface　temperature　(LST)　data　between　Landsat-8　and　Landsat-9　based　on　five　test　sites　from　different　parts　of　the　world　using　synchronized　underfly　image　pairs　of　both　satellites.　Previous　cross-comparisons　have　demonstrated　high　consistency　between　the　spectral　bands　of　Landsat-8　and　Landsat-9,　with　differences　of　around　1%.　However,　it　is　unclear　whether　this　low　deviation　will　be　amplified　in　subsequent　multiband　calculations.　It　is　also　necessary　to　determine　whether　the　difference　is　consistent　across　different　land　cover　types.　Therefore,　this　study　used　a　three-level　cross-comparison　approach　to　specifically　examine　these　concerns.　Besides　the　commonly　used　band-by-band　comparison,　which　served　as　the　first-level　comparison　in　this　study,　this　approach　included　a　second-level　comparison　based　on　the　calculations　of　several　indicators　and　a　third-level　comparison　based　on　a　composite　index　calculated　from　the　indicators　obtained　in　the　second-level　comparison.　This　three-level　approach　will　examine　whether　the　difference　found　in　the　first-level　per-band　comparison　would　change　after　the　subsequent　calculations　in　the　second-　and　third-level　comparisons.　The　Remote　Sensing　based　Ecological　Index　(RSEI)　was　used　for　this　approach　because　it　is　a　composite　index　integrating　four　indicators.　The　results　of　this　three-level　comparison　show　that　the　first-level　per-band　comparison　exhibited　high　consistency　between　the　two　satellites＇　SR　data,　with　an　average　absolute　percent　change　(PC)　of　1.88%　and　an　average　R2　of　0.957　across　six　bands　in　the　five　test　sites.　This　deviation　increased　to　2.21%　in　the　third-level　composite　index-based　comparison,　with　R2　decreasing　to　0.956.　This　indicates　that　after　complex　calculations,　the　deviation　between　the　bands　of　the　two　satellites　was　amplified　to　some　extent.　However,　when　analyzing　specific　land　cover　types,　notable　differences　emerged　between　the　two　satellites　for　the　water　category,　with　an　average　absolute　PC　ranging　from　18%　to　35%　and　an　R2　of　lower　than　0.6.　Additionally,　there　were　also　nearly　5%　differences　for　the　built-up　land　category,　with　an　average　R2　value　of　lower　than　0.7.　The　comparison　of　LST　data　between　both　satellites　also　reveals　that　the　Landsat-9　LST　is　on　average　0.24　degrees　C　lower　than　Landsat-8　LST　across　the　five　test　areas　but　can　be　0.58　degrees　C　lower　in　built-up　land-dominated　areas　and　0.42　degrees　C　higher　in　desert　environments.　Overall,　the　SR　and　LST　data　between　Landsat-8　and　Landsat-9　are　consistent.　However,　their　performance　varies　depending　on　different　land　cover　types.　Caution　is　needed　particularly　for　water-related　research　when　utilizing　both　satellites　simultaneously.　Significant　discrepancies　may　also　arise　in　the　areas　characterized　by　deserts　and　built-up　lands.