Abstract: 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 ofthe 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 ℃-0.82 ℃,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.