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Reoxidation has long been a problem when casting ultra-low oxygen liquid steel. An experimental study of the reoxidation phenomenon caused by Cr₂O₃-bearing cover flux of Al-killed steel is presented here. MgO-CaO-SiO₂-Al₂O₃-Cr₂O₃ tundish cover flux with various Cr₂O₃ contents were used to study the effects of Cr₂O₃ on total oxygen content (T[O]) and alumina and silicone loss of Al-killed steel at 1923 K (1650 °C). It was found that Cr₂O₃ can be reduced by Al to cause reoxidation, and the reaction occurs mainly within 2 to 3 min after the addition of the tundish cover flux with 5% and 10% Cr₂O₃ concentration. T[O] and Al loss increase with higher Cr₂O₃ concentration flux. Two controlled experiments were also made to investigate the oxygen transported to the steel by the decomposition of Cr₂O₃. It was calculated that when Al is present in steel, more than 90% of the reoxidation of Cr₂O₃ is caused by Al, and the rest is caused by decomposition.