In this paper,the drug sensitivity method and the tube method are adopted to study the onion's antibacterial effect to Fiexibacter Coiumnaris-the pathogen of the bacterial gill rot disease in grass carp.The result indicates that the onion has some antibacterial effect to this pathogen.Through drug sensitivity tests,it shows that the diameter of bactteria inhibiting ring is 16mm;the effect is obvious when the minimal bacteriostatic concentration is 5 or 10(uL/mL);when the concentration is below 5(uL/mL),the effect is not obvious;and the onion's antibacterial activity can't work in selective concentration range.