Titanium dioxide thin films were fabricated as hydrogen sensors and its sensing properties were tested. The titanium was deposited on a SiO2/Si substrate by the DC magnetron sputtering method and was oxidized at an optimized temperature of 850 oC in air. The titanium film originally had smooth surface morphology, but the film agglomerated to nano-size grains when the temperature reached oxidation temperature where it formed titanium oxide with a rutile structure. The oxide thin film formed by grains of tens of nanometers size also showed many short cracks and voids between the grains. The response to 1% hydrogen gas was ~2 × 106 at the optimum sensing temperature of 200 oC, and ~103 at room temperature. This extremely high sensitivity of the thin film to hydrogen was due partly to the porous structure of the nanosized sensing particles. Other sensor properties were also examined.