A hydrogen gas sensor using thermoelectric (TE) Li-doped NiO thin film with platinum catalyst film on the half of its surface was fabricated and this thin-film sensor has strong adhesion on the substrate. When this sensor is exposed to air mixed with the hydrogen gas, catalytic reaction heats up the platinum-coated surface, and then TE voltage builds up along the hot and cold region of the oxide film. The Li-doped NiO thin film was deposited by r.f. sputtering method on three different substrates and followed by thermal annealing. The catalyst Pt thin film is deposited on a half-area of NiO thin film and plays an important role for the sensing properties of this hydrogen sensor. FESEM observations indicated that the thickness of the NiO and Pt thin films for the effective catalytic reaction was approximately 720 and 75 nm, respectively. As-deposited NiO thin film possessed amorphous state and Pt thin film on this NiO film has no catalytic activity. Contrary to the as-deposited sample, thermal annealed NiO thin film has crystallized and Pt thin film on this NiO film shows usual exothermic temperature increase arisen from catalytic reaction. These results enable us to think that the catalytic activity depends mainly on the oxidation state and the crystallization degree of TE materials. In consideration of the above factors, the low resistance crystallized NiO thin film was got through the high- temperature annealing process and by using MgO(100) as the substrate. The hydrogen sensing measurement shows that the Pt/NiO thin film deposited on MgO(100) substrate could realize negligible noise level and make clear voltage signal even at room temperature.