Slag, which consists of calcium oxide, aluminium oxide, and other metal oxides, is an abundant by-product in steel-making process. It has been used as adsorbents to remove various heavy metals and the major removal mechanisms are precipitation and adsorption on the surface of metal oxide. However, the relative contribution of two removal mechanisms has not reported. In this study, the removal characteristics of copper were investigated in terms of sorption kinetics and sorption isotherms. As initial pH of solution increased, the sorption capacity of slags increased dramatically. We evaluated the relative contribution of two mechanisms in the copper removal by steel-making slag. At above pH 3.0, contribution of adsorption to overall removal of copper was less than 12%. As a result, most of copper was removed by slag as a form or copper hydroxide.