Abstract: The magnesite doped by NiO powders in different mass fractions was prepared by high-temperature calcination using natural magnesite as the raw materials in this study. The effects of NiO doping on the sintered properties, phase composition, and microstructures of magnesite were characterized by apparent porosity tester, X-ray diffractometer, and scanning electron microscope. The results show that, the NiO doping can improve the compactness of magnesite and promote the growth of MgO grain. Without the NiO doping, the line rate, bulk density, and apparent porosity of the magnesite after sintering at 1600℃ are 8.72%, 2.96 g·cm^-3, and 19.2%, respectively, and the particle size of MgO is 1.36 μm. The line rate, bulk density, and apparent porosity of the magnesite doped by NiO powders in the mass fraction of 0.75% after sintering at 1600℃ are 18.06%, 3.29 g·cm^-3, and 11.5%, respectively, and the particle size of MgO reaches to 3.81 μm. The sintered magnesite doped by NiO powders at 1600℃ can lead to the complete solid solution bewteen MgO and NiO, cause the lattice distortion of MgO, reduce the activation energy of grain growth, and consequentially promote the sintering compactness of the magnesite. But, the over-dose NiO doping may result in the partial solid solution bewteen MgO and NiO within a short time, impede the direct contact of sintered phases, obstruct the mass transfer process, and consequentially lower the sintering compactness of the magnesite.