Titanium dioxide thin films were fabricated as hydrogen sensors and its sensing properties were tested. The titanium was deposited on a $SiO_2$/Si substrate by the DC magnetron sputtering method and was oxidized at an optimized temperature of $850^{circ}C$ 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 ${sim}2{ imes}10^6$ at the optimum sensing temperature of $200^{circ}C$, and ${sim}10^3$ at room temperature. This extremely high sensitivity of the thin film to hydrogen was due partly to the porous structure of the nano-sized sensing particles. Other sensor properties were also examined.