Abstract: The highly effective depressant of dolomite from magnesite has been the focus in the field of mineral processing. Using sodium oleate (NaOL) as collector and amino trimethylene phosphonic acid (ATMP) as the depressant, the effects of these reagents on the flotation behavior of magnesite and dolomite were investigated through single and artificial mixed ore flotation tests. Furthermore, the mechanism of ATMP on the surfaces of both minerals was revealed using zeta potential, contact angle measurements, FTIR and XPS. Form the artificial mixed ore with magnesite and dolomite in the mass ratio of 4 : 1, the flotation indexes with MgO grade of 43.98%, CaO grade of 3.30%, recovery of 91.18% and separation efficiency of 91.18% could be obtained at approximately pH 10 with a reagent scheme of 20 mg/L ATMP and 60 mg/L NaOL. The results indicated that ATMP displayed an excellent depression effect on the dolomite flotation, whereas it rarely had an influence on magnesite. ATMP occupied a large number of active sites on dolomite surface through strongly interacting with the calcium sites, hindering the adsorption of NaOL on dolomite surface via electrostatic repulsion. In summary, the differential adsorption of the ATMP depressant onto magnesite and dolomite magnified the difference in hydrophobicity between them, which realizing the selective inhibition of dolomite.