ZnO nanopowders with an average grain size of 30nm were prepared by a solution combustion method with various sintering temperatures (100-1000 degrees C). The optical properties of the ZnO nanopowders were investigated in the temperature range of 14-150K in air. Based on the results of XRD, Photoluminescence, and excitation spectra, the ZnO powders showed wurtzite single phase with UV-blue light emitting. Deep level defects such as oxygen vacancies and Zn interstitials were not observed from the ZnO powders sintered up to 700 degrees C. However, these defects were observed as the sintering temperature was increased up to 900 degrees C. This seems to be due to the generation of oxygen vacancies and zinc interstitials. Furthermore, the blue light intensity was doubled when the synthesized ZnO powders were sintered at 700 degrees C. This might be very useful for high efficiency photocatalysts and the blue light emitting phosphors of displays such as field emission displays and plasma display panels.