Low grade magnesite coexisting with dolomite was purified by flotation. A process of reverse flotation to remove silicate minerals- positive flotation to concentrate magnesite was explored after the systematic process mineralogical investigation. Influence of different conditions such as grinding fineness experiment, collector experiment and combined modifiers experiment was also investigated. Open-circuit flotation test was carried out under the optimum conditions. It was showed in the result that the MgO content in concentrate was increased from 32.36% to 42.25%, the CaO and SiO2 contents in concentrate were decreased from1.08% and 17.70% to 6.73% and 0.19% respectively, but the recovery of MgO was not satisfactory.