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Vegetation-index-based spatial sharpening technologies were developed for improving the spatial resolution of thermal-infrared (TIR) images. Previous studies showed that the relationship between vegetation index and surface temperature is independent with spatial resolution. However, spatial extent is another scale factor which may affect the relationship of vegetation index and surface temperature but was neglected in the previous studies. In this letter, we investigated both of these two aspects of scale effect (spatial resolution and spatial extent) of the relationship of vegetation index and surface temperature based on two scenes of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data respectively acquired in grassland and crop-land. The result shows that the relationship of vegetation index and temperature on different spatial extents varies greatly. Therefore, previous methods which apply the relationship established on the whole image with coarse resolution to the small area (pixel size of the TIR image) with fine resolution may produce a large error. We modified the original sharpening method by establishing the vegetation-index-temperature relationship on the minimum available spatial extent (2 × 2 pixels of the TIR image) instead of the whole image. Then, the new method was tested and compared with the original method by simulated data. The result shows that the new method performs more robustly than the original method.