Abstract: Microstructures,mineral compositions,shapes and sizes of the blast furnace slag with medium titanium content were systematically studied by polarizing microscope and scanning electron microscopy. The results show that the slag consists of granular poikilitic and porphyritic microstructures,and its mineral compositions are mainly composed of melilite,followed by spinel,perovskite,titanium diopside,and a small amount of metallic iron,titanium nitride,titanium carbide and their solid solution. Titanium exists mainly in the form of perovskite,followed by titanium diopside. The perovskite,spinel,titanium nitride,titanium carbide and their solid solution in slag have great influence on the viscosity of slag.