Abstract: The non-metallic inclusions in steels were classified according to their source,chemical compositions and morphology.The hazards of non-metallic inclusions and their impacts on various properties of steel were discussed on the basis of metallurgical principles.The morphology characteristics,distribution,as well as colours under the optical microscopy bright field,dark field and polarized light of various types of inclusions were introduced.Finally,the main contents and technical points of GB/T 10561-2005 were introduced briefly.