On purpose to develop monoazo dye-decolorizing microorganism, DB51 which could highly decolorize acidic and reactive monoazo dyes was isolated and finally selected. The decolorization rates of this organism to two kinds of monoazo dyes were appeared as those of 74.5% to acid orange 6(AO6), 61.7% to acid orange 10(A010), 82.7% to reactive orange 16(R016) and 79.8% to reactive red 4(RR4) respectively. This isolate was identified as Klebsiella pneumoniae by using API 20E kit. On the temperature and pH effects to the growth rate of Klebsiella pneumoniae DB51 the most growths were appeared at the temperature of 30℃ and in pH 6.0. The growth of this isolate was inhibited an the media contained sodium chloride more than 3% of it. Effect of oxygen to the growth of Klebsiella pneumoniae DB51 was accelerated by more amount of oxygen volume. But, effect of oxygen to the decolorization rates by Klebsiella pneumoniae DB51 was decelerated by more amount of oxygen volume.