Converter steel slag was selected to study micro structural features, hydration characteristics and expansion properties of MgO, f CaO phase and RO phase in the slag using X ray diffraction(XRD), thermogravimetry differential thermal analysis(DTA TG), scanning electron microscope(SEM) and energy dispersive X ray diffraction analysis(EDX). The results show that MgO contained in converter steel slag combines with FeO and MnO to form the RO phase that is a complete MgO FeO MnO solid solution. The chemical formula is MgO·1.20FeO·0.05MnO·0.03CaO, which is stable phase tested by autoclaving. The formation of the solid solution of FeO, MnO and MgO will reduce MgO hydration activity; when the mole ratios of FeO/MgO and MnO/MgO reach to 1︰1 or 0.5︰1, RO phase is indubitable stable. The bad effect of steel slag on cement soundness is caused by hydration reaction of f CaO phase in steel slag. The f CaO phase in steel slag is solid solution containing about 0.15 mol FeO with larger size grain and smaller unit cell parameter, which is evenly distributed. Only 46.2%(by mass) of the f CaO phase in steel slag will hydrate if tested by standard boiling method, and only 70.8%(by mass) will hydrate even if boiled for 8 h. Determination of the soundness of steel slag cement should be in accordance with autoclaving test.The rapid expansion period of the steel slag cement cured in 20℃ water lasts 2 months, while the slow expansion period of that cement lasts 15 months. This long standing expansivity has the effect of compensating thermal stress of mass concrete.