Abstract: By means of T[O] and [N]measurement,metallographic observation and sample-electrolyzing,the cleanness of normal slabs and unsteady cast slabs of IF steel produced in a plant had been compared. Normal slabs have very high cleanness,where the w（T[O]） is 11 × 10^-6,w（N） is 18 × 10^-6 and the mean number density of micro inclusions is 4.0 per mm²,the fraction of inclusions over 50 μm is 2.10 mg/kg. But the unsteady continuous cast slabs contain much more inclusions. The cleanness of the first slab is worst because of the serious re-oxidation. The slabs were polluted by the slag or mold powder due to the lever fluctuation in the tudish or the mold while changing ladle or SEN. The last slab of the tudish was polluted both by the air re-oxidation and the slag entrapments. The distribution of micro inclusions shows that the maximum number of inclusion is found at the 1/4 width of the slab,while 1/2 width appears minimum amount of inclusion. Along the thickness direction,the number of inclusion near the inner and outer surfaces of the slab are obviously higher than those of central area.