Abstract: Slag oxidability, steel composition and inclusions were analyzed by slag sampling and steel sampling from the BOF endpoint to the tundish for studying the influence of ladle slag oxidability on the cleanliness of ultra low carbon steel. The results show that by ladle slag treatment after BOF tapping, T. Fe content in top slag decreases from 19.18% to 4.68% when getting in RH, indicating that the reduction of slag oxidability is obvious. The reduction of T. Fe content in slag leads to the reduction of[O] content in steel. For some heats with low T. Fe content, the average oxygen-blowing amount is large, and then the[O] content before Al deoxidization is relatively high. The relationship between the T. Fe content of RH end slag and the number of inclusions is linear, but meanwhile there is no obvious relationship between the number of inclusions after RH and the[O] content before Al deoxidization. The (CaO)/(Al₂O₃) ratio of ladle slag influences the Al₂O₃ absorption ability. The number of inclusions will be large if the (CaO)/(Al₂O₃) ratio of the slag is not reasonable. The defects of hot rolled sheets significantly decline with the reduction of slag oxidability.