Abstract: Optimum conditions for extraction of free magnesium oxide (f-MgO) from samples of steel slags using the new mixed-extractant of NH4Cl, glycol and ethanol were studied by orthogonal designing, and the following results were obtained: ① amount of sample taken: 0.3-0.5 g; ② amount of the mixed solvent of glycol and ethanol (6+1) required: 30 mL; ③ amount of NH4Cl required: keeping the ratio of nNH4Cl to nCa+Mg in the range of 18 to 25.8; and ④ treating the sample with the mixed extractant by heating to gentle boiling and refluxing for 20 min. The reaction mixture was cooled and centrifuged, the supernatant was taken, acidified with HCl (1+1), and made up to 100 mL. Aliquots of which was used for determination of (a) sum of Ca and Mg; (b) Ca only. by EDTA titration. Content of f-MgO was calculated from the difference of (a) and (b). It was shown that under the prescribed conditions, the values of recovery of f-MgO found were all greater than 97%. Four slag samples obtained from different steel plants were analyzed by the proposed method, values of RSD′s (n=5) calculated from the results of f-MgO contents were in the range of 0.79% to 2.2%.