A thermally tunable half-disk resonator (HDR) with directly-integrated metallic heater is presented. The proposed resonator is based on the structure of HDR, which allows direct electrical contacts in HDR region without causing extra loss. The metallic heater is designed to be directly integrated on the silicon devices, and single-mode operation can be retained simultaneously. Metallic heater deposited on inner side of the ring, which cannot realize before because of weakened light confinement resulting in substantial leakage and loss, guides most heat power to the waveguide. This thermal localization enhances tuning efficiency. The simulation result shows a wavelength shift of 0.855 nm under ultralow driving voltage of 0.02V, corresponding to high thermal tuning efficiency of 2.831 nm/mW. The structure possesses both the advantages of high thermal tuning efficiency and low resistance, hence requiring smaller voltage and energy to drive, desirable for optical interconnects applications. Moreover, the proposed structure also eliminates the need to use doped silicon slab for electrical contacts, as widely used in conventional directly integrated heaters. Undoped strip waveguide in HDR enables higher Q-factor and improves optical performance.