The control of inclusion in the molten steel has been widely investigated in the previous study, including deoxidation, slag modification, calcium treatment, argon blowing, vacuum treatment, and reoxidation. However, the evolution of inclusions in the solid steel during the reheating process is unclear. In the current study, the laboratory experiment was performed to investigate the formation and evolution of inclusions at heat treatment temperature. The composition of original inclusions in steel is obviously changed due to the reaction of the steel and inclusions. Moreover, the thermodynamic calculation was conducted to predict the thermodynamic condition of inclusion formation and evolution. The effects of steel composition, heat reheating time, and heat treatment temperature on the inclusion transformation rate were studied.