Abstract: The magnesia was prepared by using CoO-doped natural magnesite as raw material through two steps calcining process in this paper. Phase components and microstructures of the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results show that, the bulk density and apparent porosity of magnesia prepared by magnesite without CoO addition sintered at 1600℃ are 3.02 g·cm^-3 and 17.8%, respectively, and the particle size of MgO only is 3.66 μm. In the same sintering conditions, the bulk density and apparent porosity of magnesia prepared by magnesite doped with 0.75% CoO by mass are 3.28 g·cm^-3 and 9.4%, respectively, and the particle size of MgO reaches to 5.31 μm. After the high temperature sintering, the CoO-doped natural magnesite changes the lattice constants of MgO and makes the MgO lattice distortion, which consequentially reduces the activation energy of grain growth and promotes the sintering of natural magnesite.