Titanium and its alloys are promising materials in the dental field. However, they react easily with investment materials during casting, forming the reaction layer, resulting in changes of their mechanical property, and limiting their use. So it is necessary for us to study the surface reaction layer between titanium and investment materials and how to decrease the reaction layer. This paper studies the composition and microstructure of the surface reaction layer of titanium castings with Al2O3-based investment materials by SEM, EPMA and TEM. The result reveals that there were two layers on the surface of titanium castings, with outer layer being oxide layer including Al2O3, MgO, Al2TiO5(Al2O3•TiO2) and TixOy, and inner layer being alloy layer including Ti3Al and Ti. The formation mechanism of the surface reaction layer of titanium castings is that when molten titanium is cast into Al2O3 mold, Al2O3 dissolves into molten titanium by the form of atomic Al and O, Ti3Al and TixOy are separated out when the melt solidifies.