Together with the chemical analysis as well as XRD and DTA, effects of steam and autoclave treatments on structure characteristics of basic oxygen furnace slag (BOFS) and electric arc furnace slag (EAFS) are investigated. The results show that steam and autoclave treatments on slag contribute to the conversion of f-CaO and f-MgO to Ca(OH)2 and Mg(OH)2 ,which improve the volume stability of slag by reducing the contents of free lime and periclase, and the autoclave treatment is more effective. Meanwhile, for the utilization of steel slag, the effects on the volume stability by CaO and MgO that were wrapped in eutectic in the crystallite form should be a key factor, besides the contents of f-CaO and coarse periclase crystalline. The thermal reactivity of slag treated by steam and autoclave increased, especially the reactivity of elemental iron and wuestite. The reactivity of EAFS increased more significantly at high temperature due to its higher content of iron.