Abstract: The mechanism of submerged entry nozzles (SEN) clogging with AISI 321 stainless steel billet was investigated by analyzing the structure and composition of the SEN deposits. The results of deposits dissection indicate that nozzle clogs consist of TiN, metal and slag. Then the thermodynamic model of TiN in the steel and solidification was established. The results show that the existence of TiO₂ transition layer, high concentration product of titanium and nitrogen, and worse cleanness of the liquid steel have roles to play in the formation of deposits. Therefore the mechanism of clogging can be explained as follows. At first, a transition layer of TiO₂ forms at the SEN refractory because of the reaction of[Ti] with the silicates in the SEN refractory during casting. Subsequently the good heat transfer properties of titanium oxide layer can cause a large drop in temperature of molten steel which makes it possible for the formation of the TiN deposits, additionally low cleanness deteriorates the cast-ability of molten steel. In the end, the growth of the deposits leads to the nozzle clogging.