In the current chapter, several fundamental issues on the modification of inclusions by precise calcium treatment were theoretically and experimentally investigated. Laboratory experiments were performed to study the effect of the total calcium (T.Ca) content and the total sulfur (T.S) content in the steel on the composition, size, and number of inclusions in the steel. A reaction model for the formation of CaO and CaS during calcium treatment was established. Mechanisms for the formation and the decomposition of CaS and CaO inclusions in the steel after calcium treatment was proposed.