The antibacterial activity of the water extract from leaf of Citrus unshiu Marc was studied. The results indicated that 0.17-0.50 g·mL^-1 leaf extract from C. unshiu had different antibacterial activities to Escherichia coli （Migula） Castellani et Chalmers, Staphylococcus aureus Rosenbach, Candida albicans （Robin） Berkhout and Bacillus subtilis （Ehrenberg） Cohn, diameters of antibacterial zone were 0.14-0.38 cm. The antibacterial activity to E. coli was the best with the water extract, the diameters of antibacterial zone were 0.22-0.38 cm. With the concentration of the leaf extract increased, the antibacterial activity to the tested bacteria and yeast gradually increased.