Abstract: MgO with different nanoparticle sizes were prepared and their samples were characterized by XRD and IR spectroscopy to investigate their infrared wave absorptivities. IR spectroscopic results indicated that the absorption peak widens in the range from 400 to 600 cm^-1 and the IR wave absorptivity is improved evidently, where red shift and blue shift coexist. The blue shift is found with the decreasing particle size, then the red shift is found when an inflection point takes place in the vicinity that the particle size is around 50 nm. The crystalline field effect often results in the widened absorption peak because of the "surface atomic effect" of nanocrystals. The decreasing nanoparticle size may lead the fine structure of the spectral band of conventional lump materials to disappear, thus forming a wide and flat absorption band.