This study analyzed population density changes and taxonomic characteristics of various microorganisms, which play important roles in culture medium fermentation of Agaricus bisporus and examined changes in decompositionrelated enzyme activity secreted by these microorganisms. Various microorganisms such as thermophilic bacteria, actinomycetes, fluorescent pseudomonas spp. and filamentous bacteria are closely related to culture medium fermentation of Agaricus bisporus. The population density of these microorganisms changes and harmful bacteria disappear during thermophilic fermentation. During outdoor fermentation, Psychrobacter sp., Pseudomonas sp., Bacillus sp. and Pseudoxanthomonas sp. showed the highest percentage distribution in the different culture medium while Bacillus sp. and Psychrobacillus sp. were the most dominant after pasteurization. When analyzing the enzyme activity of microorganism during rice straw fermentation, cellulose enzymes become important at the early stage of fermentation. Increase in hemicellulose enzyme follows after decomposing carbon sources. The microorganisms that secretes these enzymes were distributed at the second and third turning stage of fermentation.