A series of cobalt oxides with different oxidation states have been prepared by using a precipitation-oxidation method and a controlled reduction method ( in situ thermogravimetry-temperature programmed reduction, STG-TPR). The gradual change in microstructure with different oxides is studied by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results show that high-valence cobalt oxide is hexagonal with an elongated shape, Co3O4 is spinel with hollow spheroidal shape and CoO is a face-centered cubic structure, respectively.