A　tandem　cross　calibration　between　ASTER＇s　and　Landsat　ETM+＇s　thermal　data　was　carried　out.　Three　simultaneous　TIR　image　pairs　of　the　two　sensors,　each　of　which　imaged　on　common　ground　targets,　were　used　to　quantitatively　compare　the　thermal　quality　of　the　two　sensors.　The　comparison　has　been　made　between　Landsat　ETM+　band　6　and　ASTER　bands　13　and　14.　The　result　shows　that　an　excellent　agreement　between　the　at-sensor　temperatures　obtained　from　both　sensors　for　the　TIR　band(s).　A　very　strong　positive　correlation　between　the　two　sensors＇　TIR　data　has　been　revealed.　However,　the　at-sensor　temperature　derived　from　ASTER　thermal　images　is　0.66°C-0.82°C,　higher　than　that　derived　from　ETM+　thermal　images.　In　addition,　the　ASTER　TIR　images　have　higher　thermal　information　gain　and　more　continuous　measurement　on　ground　thermal　targets.