Abstract: In order to control the shape of inclusions in steel liquid and determine the rare earth Y and Ce and O,S reaction sequenceand the formation of the rare earth inclusions,the rare earth degeneration inclusion in E36 ship plate steel had been investigated through thermodynamic calculation and experimental study,and the typical inclusion had been observed using scanning electron microscopyand EDS.It is found out that Ce has more tendency to react with O and S than Y.The sequenceof the rare earth inclusions formed are Re₂O₃,Re₂O₂ S,Re₂S³.The dominant rare earth compound inclusions are sizes at~15 μm and~50 μm.Compared with the inclusion of steel without rare earth addition,the inclusion size of RE treated steel reduces by 20~55 μm and its shape are more globular.Rare earth oxides mostly locates on the 1/4 thickness of upper surface and rare earth sulfur oxides locates on the 1/2 thickness.Rare earth addition can improve the morphology and size of inclusion,and thusimprove the performance of the steel.