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It is very necessary to validate MODIS land surface temperature (LST) for its application, especially in the arid and semi-arid regions. In this study, the Terra and Aqua MODIS 1km daily LST products (MOD/MYD11A1) are validated using ground based longwave radiation observation. The longwave radiation ground measurements during 2008 to 2009 were collected from four automatic weather stations in the Heihe River Basin. In this validation process, the land surface broadband emissivities of the validation stations were obtained from ASTER Spectral library. Then the ground-measured LSTs of validation stations were converted from surface longwave radiation based on Stefan-Boltzmann's law and thermal radioactive transfer theory. The validation results indicated that: except for DYKGT station, the mean bias was less then 1K and the mean absolute error (MAE) range was about 2-3K; MYD11A1 LSTs from Aqua have larger biases, MAEs, and RMSDs than that of MOD11A1 LSTs from Terra in most cases. The comparisons with ground measured LSTs show that the MAEs and RMSDs from daytime MOD/MYD11A1 comparisons are larger than that from nighttime MOD/MYD11A1 comparisons.