In this paper, we investigated TiO₂ as gate dielectric to achieve the large dielectric constant. The ultra high-k value over 30 was obtained by Capacitance-Voltage measurement of Al/Ti/TiO₂/Si Metal-Insulator-Semiconductor (MIS) capacitor. Among as deposited, rapid thermal annealing (RTA) at 750 °C and 1000 °C, the RTA at 750 °C showed the lowest gate leakage current. It implies that TiO₂ has optimum RTA temperature having the lowest leakage current. When TiO₂ is annealed at 750 °C, the phase of TiO₂ changes to anatase and interfacial layer between TiOx and Si was formed. While TiO₂ is annealed at 1000 °C, the phase of TiO₂ changes to rutile and diffusion of silicon atoms was clearly observed and it causes the silicide formation. Based on measurement data, we proposed the energy band diagram of Al/TiO₂/Si MIS capacitors. This diagram shows that the energy band gap of RTA at 750 °C is expanded while that of RTA at 1000 °C is contracted. In addition, TiO₂ with RTA at 550 °C was tested to confirm leakage current and it shows lower leakage current than RTA at 750 °C as we expected. This result confirmed that optimum RTA temperature of TiO₂ would exist under 750 °C.