One thermophilic anaerobic oil-degrading bacterium was successfully isolated from reservoir brine of Yabase oilfield, INPEX Corp., Akita, Japan. The potential of the isolated bacterium as a candidate for Microbial EOR (MEOR) was estimated in this study. This bacterium was identified as Petrotoga sp. by DNA sequencing analysis. The isolated bacterium can degrade long chain hydrocarbons in crude oil into shorter chain hydrocarbons in reservoir brine containing 2 (v/v) % of crude oil as a carbon source and 0.1 g/1 of yeast extract as a nitrogen source. As a result, the viscosity of crude oil decreased. The isolated bacterium can form their bacterial colonies on a solid medium exclusively under the presence of CO2 in gas phase. In addition, the results of GC-MS analyses of crude oil showed that the isolated bacterium can degrade longer chain of n-alkanes more selectively under 10 % CO2 atmospheric condition. These results show that CO2 stimulate the growth of the isolated bacterium and selective degradation of longer chain of n-alkanes by the bacterium. The suitable nitrogen source for the isolated bacterium was evaluated by reduction of viscosity of crude oil. Ammonium nitrate, urea, and yeast extract were evaluated for the nitrogen source and added into the medium containing crude oil with 2 (v/v) %. The highest growth rate of the bacterium was observed in the medium containing yeast extract. Viscosity of crude oil reduced by 40 % of its original viscosity in this case while ammonium nitrate, urea and non-nitrogen source addition gave 38 %, 35 % and 12 % oil viscosity reduction after 3 weeks incubation. 0.05 g/1 of yeast extract is enough for the growth of the isolated bacterium. The isolated bacterium can grow under high salinity condition such as 90 g/1 of NaCl and it can grow under the temperature between 50 to 80 °C. These results show that the isolated bacterium can be applied to wide range of reservoirs.