Purpose　The aim is to study the inhibiting effect s of nineteen amino acids to E.coli,St.aureus and B.Subtilis.Metho ds　The qualitative inhibition of nineteen amino acids on E.coli ,St.aureus and B.Subtilis was determined using filter papers treated by saturated amino acids placed on the bacteria medium dish.The quantitative inhibi ting effects of the amino acids which were proved to have inhibition to these th ree bacteria were determined in the bacteria LB medium with different concentrations of amin o acids.Results　It was indicated that cysteine was used to in hibit St.aureus.The best inhibiting time was 6 hours,and the optimum of concentrat ion was 0.625% in which the inhibiting rate was 92.62%.Conclusion　 Cysteine could inhibit St. aureus.