Mushroom is cultivated as one of the major economical crops in many areas in Korea. The total production has steadily increased from approximately 198,563 M/T in 2009 to 208,941 M/T in 2012. Several fungus are known as the causal agents of diseases of the cultivated button mushroom (Agaricus bisporus) and oyster mushroom (Pleurotus ostreatus). Trichoderma harzianum is the causal agent of brown blotch disease of commercial mushrooms. Colonization of mushroom caps by the bacterium results in development of brown or cream lesions on pileus and stipe. These lesions are slightly concave spots and can be round or spreading. Antagonists against Trichoderma harzianum, CAM33 were selected and their control efficacy of green mould disease was investigated in this study. The CAM33 strain was selected as antagonistic bacterium by inhibition zone method and it was identified as Bacillus methyrotrophicus. by the cultural, morphological and physiological characteristics, and analysis of the 16S rDNA. The isolated bacterium is saprophytic but not parasitic nor pathogenic to cultivation mushroom. The isolated bacterium for Trichoderma harzianum cell, was sufficient for inhibition in vitro. Inoculation of the isolated bacterium prevents the development of bacterial disease in Trichoderma harzianum. Control efficacy of browning disease of strain CAM33 treatment was 77% on Agaricus bisporus. The optimal culture medium for the antagonistic bacteria growth was determined as follows: 3.0% Saccharose, 1.5% Soytone, 1.0% (NH4)2HPO4, 10 mmol MgSO4, and 2.0% Glutamic acid at pH 6.0 at 25°C. The suppressive bacterium may be useful in future for the development of biocontrol system and the construction of genetically modified edible fungi resistant to the disease caused by Trichoderma harzianum.