In order to improve the quality of the products and the pure degree of the steel; the type, quantity, size, composition and distribution of the nonmetallic inclusions present in continuous cast slabs made by Weiyuan Iron and Steel Co. , Ltd. are investigated with optic microscope, SEM, XES and Electroanalysis. Moreover the original of the inclusions is discussed. The result shows the inclusion in the continuous cast slab is mainly the spherical silicate inclusion which is smaller than 10 um. There is a small amount of polygonal inclusions, and there is the single-phase one and heterogeneous inclusion too; And the inclusions assemble in 1/3 position and in 1/4 at board base thickness and width direction. The inclusions are ineral, which origin from the deoxidizing process of molten steel.