Abstract: The influences of slag-metal reaction and calcium treatment on inclusion modification in high-strength low-alloy steel were studied by means of Thermo-Calc thermodynamic software and ASPEX automated scanning electron microscope. The results of industrial trials show that by increasing the basicity, reducibility and w(CaO)/w(Al₂O₃) of slag, Al₂O₃ inclusions are partially modified to CaO-MgO-Al₂O₃ complex inclusions nearby the liquid phase zone at 1 600℃, as well as a few MgO·Al₂O₃ spinel. On the basis of this change, the purpose of inclusion modification is still achieved with the addition of CaSi-wire decreasing from 800 m to 300 m for a heat.