Inclusions in steel samples collected 0.5, 2, 5, 10 and 20 min after calcium addition were investigated in Low Carbon Aluminum Killed steel (LCAK steel) with ultra low sulfur content (0.04–0.065% Al, 9 ppm S). It’s found that inclusions change from CaO to CaO–CaS and finally to CaO–Al₂O₃–CaS with time. Al₂O₃ in inclusions linearly decreases by increasing T.Ca/T.O of the steel and disappears when T.Ca/T.O exceeds 3. CaS/CaO of the inclusions linearly increases by increasing S/T.O of the steel. Contents of CaO, Al₂O₃ and CaS can be estimated through T.Ca, S and T.O. CaO–CaS clusters are found at 5 min. The mechanism of inclusion evolution and formation of CaO–CaS clusters are discussed.