Abstract: In order to detect the effect of Mg on the inclusions in 20 CrMo gear steel,the smelting experiments of Mg addition to gear steel were carried out by using a high-temperature tube electric resistance furnace.Combined with the methods of SEM and OM,the morphology and composition of inclusions in this steel were characterized.And the change mechanisms of inclusion compositions in the steel were also investigated with thermodynamics under various Mg additions.Experimental results showed that the density of inclusions amount in steel was in range of 40～65 mm~2.With Mg content of 8×10^-6,the equivalent diameter of inclusions increased from 2.93 to 3.28 μm,whereas,with Mg content of（21～38）×10^-6,the equivalent diameter of inclusions was in range of 2.64～2.67 μm.With raising the Mg content in the steel,the percentage of single MnS inclusion reduced from 26.3% to 2.2%,while the percentage of oxides increased from 44.8% to 84.8%.Thermodynamics calculation indicated that the Mg with content of（2.63～7.98）×10^-6 could completely modify the Al₂O₃ into MgO·Al₂O₃ spinel; when Mg content is above 7.98×10^-6,MgO inclusions were generated.It is concluded that the proportion of composite inclusions at 8 ppm of Mg content was the maximum,whereas the proportion of oxides was relatively lower,which is beneficial to improving the mechanical properties of gear steel.