The measurements of air temperature and air humidity were utilized to analyze the diurnal variation of atmospheric emissivity over heterogeneous surfaces and to calculate downward atmospheric longwave irradiance, from June 5th to July 6th, 2004, in Xiaotangshan area, Beijing. Moreover, sky radiometric temperature at 37 and land surface radiometric temperature with thermal infrared thermometers were employed to study the differences between surface radiometric temperature over heterogeneous surfaces without being calibrated and calibrated with standard blackbody source and the differences between surface “true” temperature, which were calculated with the downward longwave irradiance and surface emissivity, and surface radiometric temperature, which were calibrated by standard blackbody source. The results may be served as scientific reference to invert land surface temperature with remote sensing and to study land surface energy balance. The results indicated: (1) the differences of surface radiometric temperature between without being calibrated and being calibrated by standard blackbody sources ranged from 0.1 to 1℃;（2）the diurnal variation range of atmospheric emissivity ranged from 0.75 to 0.85; (3) the differences of downward atmospheric longwave irradiance between being calculated by air temperature and air humidity and being measured by pyranometer were small with the average mean error being 3.1%, while those of downward atmospheric longwave irradiance between being calculated by sky radiometric temperature at 37° with thermal infrared thermometer and being measured by pyranometer were larger, with the average mean error being 38.1%; (4) the differences between surface true temperature and surface radiometric temperature calibrated with standard blackbody ranged from 0.2℃ to 1.5℃ over various surfaces under variable sky conditions, and the higher surface radiometric temperature was, the larger the differences were. Therefore, the differences had the diurnal variation rule.