Abstract: To analyze the behaviour of inclusions in Al-killed steel during LF refining and the effect of Ca -treatment, samples were taken at 3 different periods which were 3 minutes after refining, after white slag formation and after Ca-treatment. The number, shape and composition of the non-metallic inclusions were analyzed and contrasted. The results showed that Ca-treatment was effective in improving the cleanliness of steel and modifying the inclusions. Spheroidizing ratio of inclusions increased obviously after white slag formation and increased more after Ca-treatment. Most of inclusions were Al₂O₃-SiO₂-MnO and Al₂O₃ after 3 minutes refining. After white slag formation the inclusions transformed to Al₂O₃-CaO(CaS)-SiO₂, then transformed to Al₂O₃-CaO after Ca-treatment.