The influence of different rare earth amounts on microstructure and mechanical properties of 5Cr21Mn9Ni4N steel was studied. The microstructure and composition and shape of inclusions under as-cast, as well as hot rolling state of 5Cr21Mn9Ni4N steel were observed under metallographic microscope and SEM. The results indicate that dendrite of as-cast condition is refined obviously, the extent of segregation is lightened, dimension of inclusions is lessened, and the shape of inclusions is changed from polygon to ellipse when appropriate amount of mixed rare earth is added. Appropriate rare earth also can lighten the extent of hot rolling texture and improve distribution of carbide under hot rolling state. The results of room temperature and 500 ℃ tensile test show that room temperature tensile strength is firstly improved and then appreciably descended. The elongation percentage decreased with the increase of rare earth amount. Appropriate rare earth amount can improve the high temperature tensile strength and ductility of 5Cr21Mn9Ni4N. Fracture surface observation and energy spectrum analysis indicate that complicated rare earth inclusions appear in the fracture surface when 0.10% (mass fraction) rare earth is added in experimental steels, and tough nests of fracture surface become fine without big hole. The influence of different rare earth amounts on microstructure and mechanical properties of 5Cr21Mn9Ni4N steel was studied. The microstructure and composition and shape of inclusions under as-cast, as well as hot rolling state of 5Cr21Mn9Ni4N steel were observed under metallographic microscope and SEM. The results indicate that dendrite of as-cast condition is refined obviously, the extent of segregation is lightened, dimension of inclusions is lessened, and the shape of inclusions is changed from polygon to ellipse when appropriate amount of mixed rare earth is added. Appropriate rare earth also can lighten the extent of hot rolling texture and improve distribution of carbide under hot rolling state. The results of room temperature and 500 ° C tensile test show that room temperature tensile strength is previously improved and then appreciably descended. rare earth amount. Appropriate rare earth amount can improve the high temperat ure tensile strength and ductility of 5Cr21Mn9Ni4N. Fracture surface observation and energy spectrum analysis indicate that complicated rare earth inclusions appear in the fracture surface when 0.10% (mass fraction) rare earth is added in experimental steels, and tough nests of fracture surface become fine without big hole