Abstract: High cleanliness is required for low carbon aluminum killed (LCAK for short) steel which is widely used for production of cold rolled sheet. But, head slab cleanliness is seriously deteriorated by the unsteady casting condition at casting start stage. Cleanliness of LCAK steel head slab was systemically investigated on aspects of slab composition and non-metallic inclusions under the present casting conditions. Besides, factors affecting the cleanliness of head slab were discussed. A cast starting number is defined.The results show that the head slab cleanliness is unstable until the number reaches 0.17, which can help slab quality classification. The main factor deteriorating head slab is the reoxidation in tundish. Besides oxygen in the air, more extent of reoxidation is caused by the refractory and covering slag in tundish. Several measures of improving head slab cleanliness were put forward as well, which can provide certain reference on plant operations.