In order to analyze the sources of inclusions in ultralow carbon IF steel, the tracer was added after converter tapping and tundish flux to determine the source of inclusions. Steel samples and slag samples were taken in the relevant processes, and the morphology and composition of inclusions were observed and analyzed by SEM. The results showed that when w（TFe） in the top slag were controlled in 2.54%4.49 % and 4.56%5.23% for the RH station and RH end, respectively, and the w(CaO)/w(Al2O3) ratio of the RH top slag was controlled in 1.201.93, the modification effect of the top slag was good. By analyzing the normal slab, it was found that there was a small amount of Ba and no Ce. The transition slabs between the sixth and seventh heat were analyzed. It was found that there was some Ba and a small amount of Ce. w（TO） of normal slabs in the sixth heat was 0.001 4%, w（TO） of the transition slabs in the sixth and seventh heat was 0.001 6%, w（TO） of the normal slabs in the seventh heat reduced to 0.001 5%, which indicated that w（TO） in the normal slabs was lower than that of the transition slab.