For the detection of contaminated liquid spawn, we selected suitable medium, indicator and developed method of diagnosis. The growth of pathogenic bacteria, Pseudomonas sp., and fungi, Trichoderma sp., in YPL media was better than in PDA and NA. In addition, the changes of color and absorbance of media were obviously showed when contaminated liquid spawn by pathogenic bacteria and fungi was incubated on YPL including phenol red for 48 hour at 25 oC. The color of YPLP after incubating of infected liquid spawn by Pseudomonas sp. and Trichoderma sp. were changed from orange to red and to scarlet, respectively. Whereas, the color of YPLP after incubation of only Pleurotus ostreatus indicated yellow at liquid spawn. Therefore, it is possible to easily distinguish contaminated liquid spawn by color of change in YPLP.