Investigations were made both in laboratorial and industrial scales on formation of non-metallic inclusions with relatively lower melting temperatures to improve the fatigue property of the special steels which contained extra low oxygen.It was found in laboratory studies that steel/slag reaction time largely affected non-metallic inclusions in steel.With the slag/steel reaction time increasing from 30 to 90 min,inclusions of MgO-Al2O3 spinel were gradually changed into CaO-MgO-Al2O3 system inclusions which were surrounded by lower melting temperature softer CaO-Al2O3 surface layers.By using high basicity and as much as 41 mass% Al2O3 refining slag,the ratio of the lower melting temperature CaO-MgO-Al2O3 system inclusions was remarkably increased to above 80%.In the industrial experiments,it was found that the inclusions changed in the order of “Al2O3 →MgO-Al2O3 system→CaO-MgO-Al2O3 system” in the secondary refining,and the change from MgO-Al2O3 system to CaO-MgO-Al2O3 system took place from the outside to the inside.The diffusion of CaO and MgO inside the layer of CaO-MgO-Al2O3 was considered as the controlling step of the inclusion transfer.Through LF and RH refining,most inclusions could be transferred to lower melting temperature CaO-Al2O3 and CaO-MgO-Al2O3 system inclusions. Investigations were made both in laboratorial and industrial scales on formation of non-metallic inclusions with relatively lower melting temperatures to improve the fatigue property of the special steels which contained extra low oxygen. It was found in laboratory studies that steel / slag reaction time mainly affected non-metallic inclusions in steel. With the slag / steel reaction time increasing from 30 to 90 min, inclusions of MgO-Al2O3 spinel were gradually changed into CaO-MgO-Al2O3 system inclusions which were surrounded by lower melting temperature softer CaO-Al2O3 surface layers.By using high basicity and as much as 41 mass% Al2O3 refining slag, the ratio of the lower melting temperature CaO-MgO-Al2O3 system inclusions was remarkably increased to above 80% .In the industrial experiments, it was found that the inclusions changed in the order of “Al2O3 → MgO-Al2O3 system → CaO-MgO-Al2O3 system” in the secondary refining, and the change from MgO-Al2O3 system to CaO-MgO-Al2O3 system too k place from the outside to the inside. The diffusion of CaO and MgO inside the layer of CaO-MgO-Al2O3 was considered as the controlling step of the inclusion transfer. LF and RH refining, most inclusions could be transferred to the lower melting temperature CaO-Al2O3 and CaO-MgO-Al2O3 system inclusions.