Abstract: When the ratio of RE/S in steel reachs to a certain value, for example RE/S =2.2 in 16Mn steel, RE/S = 2.0 in 20MnVB steel, RE/S = 1.5 in MnTiB steel, RE/S = 1.2 in 09CuPTi steel, the best effect purifying malden steel is obtained. The modification of oxides by RE addition is important to the improvement of fatigue resistance and it depends on the ratio of a_co·a_S/a_Al·a_o and a_Ce/a_Al in steel. When RE/S ratio is 2.5, 1.8 or 1.5 in steel 16Mn or 20MnVB, 25MnTiB and 09CuPTi respectively, the all sulfide and oxide modification will be achieved. Therefore the optimal ratio range of RE/S in steel must be controlled.