Application of a thermophilic anaerobic bacterium which degrades long-chain alkanes of crude oil preferentially and induces oil viscosity reduction to MEOR in our target oilfield was evaluated in this study. Although the salinity of formation-water in our target reservoir is approximately 10 % which is considerably higher than the optimum salinity for the bacterium, the bacterium can grow well and induce oil viscosity reduction in the formation-water which was diluted with sea-water whose salinity was approximately 4 % and contained yeast extract as a nitrogen source. Oil viscosity was reduced to 70 percent of its original viscosity after two-week incubation of the bacterium in the culture medium consisting of sea-water supplemented with 1.0 g/L of yeast extract. The performance of MEOR using this bacterium was evaluated by numerical simulation using two dimensional oil-water two-phase flow model. This model consists of 6 compositions: degraded oil, undegraded oil, brine, bacterium, sodium chloride and yeast extract. Undegraded oil and yeast extract are carbon source and nitrogen source for the bacterial growth respectively. Growth rate of the bacterium is calculated by Monod equation depending on the variables of the concentration of yeast extract and sodium chloride. Conversion of undegraded oil into degraded oil is depended on the proliferation of the bacterium. Growth of the bacterium is stopped by deficiency of either yeast extract or undegraded oil. Oil viscosity is reduced as the percentage of degraded oil in oil phase increases. Residual oil saturation is improved by oil viscosity reduction and enhancement of oil recovery can be obtained. According to the numerical experiments, growth of the bacterium and oil viscosity reduction were found only around the injection well because the bacterium consumed whole yeast extract around there. Recovery factor therefore can be increased by increase of injection volume of yeast extract. As a result, enhancement of oil recovery reached to 5 % by 1.0 pore volume injection of sea-water containing the bacterium and 1.0 g/L of yeast extract. 100 bbl of oil was recovered additionally by using 1.0 tons of yeast extract in that case.