According to the thermogravimetric analysis of cobalt oxalate, different micropowder of Co3O4 was prepared from cobalt oxalate by thermal decomposition at different condition, which was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results showed that Co3O4 prepared at the low temperature has a normal crystalliform structure of Co3O4 and it was arborization, while Co3O4 prepared at the high temperature, especially at two stage heating-up, had the crystalliform structure of activity Co3O4 which was spherical shape and its size is from 0.1 urn to 1um.