By adopting a room-temperature grinding of the zinc acetate-oxalic acid powder mixture and a subsequent thermal decomposition reaction, agglomerated ZnO nanocrystals with diameters of 15-40 nm were prepared. By adding NaCl in the whole process, well-dispersed ZnO nanocrystals with diameters of 3-4 nm, showing an obvious blue shift due to the size quantization effect, were prepared.