The thermal decomposition process of zinc hydroxide carbonate was studied by thermal gravimetric analysis (TG), differential thermal analysis (DTA) and derivative differential thermal analysis (DTG). It was shown that water and carbon dioxide were formed at the same time and zinc hydroxide carbonate decomposed into zinc oxide. The mechanism model was deduced and was inferred as random nucleation mechanism (Aurami equation II ). The structure and morphology of nanometer-sized zinc oxide by using thermal decomposition method were investigated by means of XRD and TEM. The study indicated that the well-crystallized and dispersed, narrow-sized, nanometer-sized zinc oxide of about 8-10nm was available by using this method.