Abstract: Cleanliness of IF steel during casting process was systematically investigated. The research on T[O]in RH-tundish-slab shows that from tundish to slab,average contents of T[O]decrease from 31× 10^-6 to 20 × 10^-6while average contents of nitrogen increase from 16.5 × 10^-6 to 19.6 × 10^-6. The analysis on inclusions in tundish for the first three heats of a casting batch shows that the number of inclusions with diameters smaller than 2 μm reduces significantly to no more than 50 percent of the initial amount. Original position analysis(OPA) on surface inclusions in head slab,tail slab and normal slab shows that the content of Al series inclusions in head slab is apparently higher than those in normal slab and tail slab; the distribution tendency of Al series inclusions in different depth in inner arc is consistent with that in outer arc; where the distance to the surface is 1.6 mm,the content of Al series inclusions the lowest; where the distance to the surface is above 1.6 mm,the content increases while the distance above 3.2 mm,the content tends to decrease.