A steelmaking slag is one of the main by-products of iron and steel. The slag has been reused as aggregate for road and civil constructions. However, the slags may contain free magnesia (f-MgO) that can cause expansive self-destruction by reactions with water and carbon dioxide in the air. It is thus very important in evaluating the quality of the slags as raw materials to develop an accurate and precise method for determination of f-MgO in the slag. However, a reliable analytical method for determination of f-MgO in slag samples has not been reported yet. In the present work, we developed a method for determination of f-MgO in the slags by hydration of f-MgO with microwave digestion followed by thermogravimetry (TG). The microwave assisted hydration with hot water over 100°C makes f-MgO to rapidly and completely convert into Mg(OH)₂, the amount of which, in turn, can be determined by TG. The developed method was successfully applied to the determination of f-MgO in synthetic and real steelmaking slag samples.