With the evolution of the steel grades that demands specific applications and highquality requirements, grades with low sulfur content have had an expressive increasingly demand. Based on these facts, this work aims to evaluate the properties and characteristics of the slag formed in the steel desulfurization step in the ladle, after the steel tapping from converter to ladle, for a steel grade with a target sulfur content (S) of 30ppm, and relate them to the efficiency of desulfurization, which measures the ability to remove sulfur. The evaluation of the slag properties was performed with the aid of a computational thermodynamics' software, for a better understanding of the phenomena that occur at the metal-slag interfaces in the steel refining process, as well as in the evaluation of important properties for these processes. The Steel Desulfurization Factor (SFDeS) parameter developed by Pezzin [1] was adapted for the case where some slags do not have a solid CaO fraction. The results confirmed that the most efficient slags for steel desulfurization are those with an optimal solid CaO content and that the Desulfurization Factor can be used to evaluate and predict the desulfurization efficiency for the studied slags.