Abstract: The behavior of the inclusion in 20MnSi continuously cast billet is studied by large sample electrolysis, metallographical observation, SEM and EDS etc. The amount, size, distribution, composition and source of the inclusion are investigated. The results show that the unreasonable tundish structure and the unstable casting operation are the mainly causes for the low cleanliness of the billet. This result could be useful to optimize the production technology and improve the quality of 20MnSi continuously cast billet.