Nanocrystalline ZnO was prepared with ZnCl2·2H2O and (NH4)2CO3 as raw materials by direct precipitation method.The precursor was proved to be [Zn5(OH)6(CO3)2] by TG-DTG-DTA and IR analysis.This precursor was calcined at 300℃ for 1,2 and 3 hours respectively,and then the nanocrystalline ZnO of different grain size were obtained.The nanocrystalline ZnO was characterized using X-ray diffraction (XRD),TEM and Brunner-Emmett-Teller method (BET).Experimental results for nanocrystalline ZnO showed that the minimum size was about 8nm,the maximum was about 15 nm and the mean grain size was 12 nm,the surface area was 80.56 m2/g and the purity was 99.9% when the precursor was calcined at 300℃ for 2 h.