Bacillus amyloliquefaciens M27 isolated from the cotton-waste compost for cultivation of oyster mushroom (Pleurotus ostreatus). B. amyloliquefaciens M27 is a biocontrol agent with antagonistic activities against a wide range of fungal pathogens. The filtrate of the isolate cultured in LB media was treated to cucumber plants in July, October and December before cucumber powdery mildew occurred. When 10-fold diluted filtrate of the isolate was treated, control efficacy was 88.9~98.9% in the treated seasons. The result showed that the culture filtrate of Bacillus sp. is very effective to control powdery mildew of cucumber during the seasons of the disease development.