Under different content of blast furnace slag and steel slag powder, cements were mixed to investigate the effect of dosage of these two mineral admixtures on strength, autoclave expansion and the relationship between strength and volume fraction of pore. The results indicated that the ratio of clinker content to ground granulated blast furnace slag(GGBS) content is the crucial factor for compressive strength of mortars incorporated GGBS and steel slag at 28d. With different dosage of steel slag, the compressive strength of 1:1 mixes of clinker and GGBS has the maximum strength. With the steel slag mixed in cement, the porosity of cement pastes was increased. With the blast furnace slag mixed in cement, the porosity and pore size of cement pastes was decreased. Compressive strength of mortars was closely related to the content of pore in the sizes greater than 50 nm at 28d. Incorporating GGBS can significantly decrease the autoclave expansion of cement deduced by blending steel slag.