Abstract: The mineral structure of blast furnace slag with different mass percent of TiO₂ （7%~16%）was studied by means of polarization microscope. The results indicate that the microstructure of blast furnace slag is mainly porphyritic or porphyritic-like texture. Phenocryst minerals mainly consist of the rich titanium diopside and melilite,and its matrix is composed by glass,titanaugite,perovskite,and a small amount of metallic iron,titanium nitride,titanium carbide and their solid solution. As the mass percent of TiO₂ increases,the content of the rich titanium diopside and titanaugite reduces first and then increases,while the content of the melilite increases first and then decreases. When the mass percent of TiO₂ reaches to 12%,the content of titanium nitride,titanium carbide and their solid solution increases. These highmelting point compounds resulted in higher viscosity and melting temperature of blast furnace slag,and the liquidity of blast furnace slag also became worse.