The laboratory experiments of Al₂O₃ inclusions modified by calcium treatment in linepipe steels with the S content of 30 ppm and 310 ppm were performed at 1873 K. Particularly, samples were taken before calcium treatment ( 1 minute before the calcium addition) and at various times (typically 1 minute, 10 minutes and 30 minutes) after calcium treatment to study the transient inclusions evolution during calcium modification. Traditional modification mechanism of Al₂O₃ inclusions is that Al₂O₃ inclusion is directly modified by dissolved calcium or indirectly modified via CaO formed from calcium and oxygen. In the current study, after Ca–Si alloy power addition, CaS outer layer were promptly formed on the angular Al₂O₃ inclusion. With the reaction of the CaS outer layer and the Al₂O₃ core or [O], the Al₂O₃ inclusion was modified to a spherical liquid calcium aluminate. Moreover, the equilibrium curve of Ca–O and Ca–S, the stability diagram of inclusions and equilibrium precipitation of inclusions during solidification in linepipe steels were calculated to study the formation of inclusions.