Abstract: The instantaneous morphological transition of triangular Al₂O₃ particles with various sizes in the molten Ca-treated steel was observed using a confocal scanning laser microscope at the steelmaking temperature. The composition of inclusions at different times was analyzed using scanning electron microscopy–energy dispersive spectroscopy. The shape evolution of particles was characterized by the shape parameter of overall regularity. It was found that the overall regularity of particles gradually increased with time during the calcium treatment. The geometry of particles tended to be more rounded and regular as the overall regularity increased during the modification process. An empirical formula was proposed to predict the composition of inclusion particles based on their overall regularity during the calcium treatment. When the CaO/Al₂O₃ mass ratio in the particle increased to 0.451, the particle was considered an ideal spherical calcium aluminate inclusion with the overall regularity of 1. Smaller particle sizes promoted the transformation of Al₂O₃ inclusions to spherical calcium aluminates during the calcium treatment.