<p>Industrial trials and thermodynamic calculations were carried out to investigate the effect of steel cleanliness on the composition of inclusions both in the molten steel and in the solidified steel of Al-killed Ca-treated low-sulfur steels. The composition of inclusions changed significantly at the Ca treatment stage with the modification of Al₂O₃ into calcium aluminate and a slight decrease in the inclusion size and at the casting stage with the transformation of CaO into CaS with an increase in the inclusion size. Based on experimental results and thermodynamic calculations, the Al₂O₃ content in inclusions in the molten steel showed a nearly linear inverse relationship with the total calcium (TCa)/total oxygen (TO) in the steel when TCa/TO &lt; 3 and kept less than 10 wt.% under TCa/TO &gt; 3. The CaO content in inclusions firstly increased until the TCa/TO reached 1–2 and then slightly decreased with the increase in TCa/TO. The CaS content in inclusions was less than 10 wt.% when TCa/TO &lt; 1.5 and increased with the increase in the TCa/TO and total sulfur (TS) content in the steel. After solidification and cooling, the CaO/CaS in inclusions increased with the increase in the TO/TS and TCa contents in the steel; however, the CaO/CaS was less than 1.0 regardless of the TCa content when TO/TS &lt; 1. Accordingly, the composition of inclusions after solidification could be adjusted by controlling the cleanliness and calcium content of the steel, thereby adjusting the property of inclusions.</p>