Steelmaking slags have been widely used as aggregate for road and civil engineering. The slags sometimes expand as a result of hydration, which may cause evolution of cracks at surfaces of roads when used as aggregate. Free magnesium oxide (free-MgO) in the slags is a potential cause of the expansion as well as free calcium oxide is. Thus, analytical methods to determine the free-MgO in the slag have been required for the accurate evaluation of the expansion. We present an accurate method to determine the free-MgO in steelmaking slags based on the combination of chemical extraction and thermogravimetry (TG). Free-MgO and magnesium hydroxide (Mg(OH) 2) in steelmaking slags are dissolved into an ethyleneglycol solution containing iodine and ethanol when heated. The amount of the magnesium species dissolved in the solution is determined by inductively coupled plasma atomic emission spectrometry. The amount of Mg(OH) 2 in the slags determined by TG independently is subtracted from that of the dissolved in ethyleneglycol to yield the amount of free-MgO. The established method can quantify less than 0.1 wt% free-MgO in steelmaking slags.