Abstract: It is found that Ca-treated liquid steel can be cast at lower superheat. In order to ascertain the reason that the castability of Ca-treated liquid steel is improved and to find optimal range of calcium content,behavior of calcium in liquid steel was studied in terms of relationship between calcium content and solid ratio of inclusions.By analyzing the composition of inclusions in Ca-treated steel samples of different calcium content,especially calcium aluminate or calcium aluminum silicate inclusions,the relationship between calcium content and solid ratio of inclusions was obtained by means of Al₂O₃-CaO-SiO₂ phase diagram.For making well effect of Ca treatment and improving fluidity or castability of Ca-treated liquid steel,the optimum calcium content should be（17～23）×10^-6.