Abstract: In this paper,non-metallic inclusion in improved 18NiNb maraging steel produced by vacuum induction melting and metallographic method.were determined and rated.The results indicated that removing of the larger size of the sulfide and single-particle inclusions in steel by ESR was efficient and the inclusions in steel produced by ESR were mainly small oxide inclusions,of which number decreased significantly,of which distribution were diffuse,and the 18NiNb alloy steel ingot quality was further enhanced.