The effects of shape, crystallinity and specific surface area of ZnO powder on antibacterial activity were studied by measuring the change in the electrical conductivity with bacterial growth. From the results, it was clarified that the antibacterial activity increased with the increase of powder concentration in the physiological saline. The antibacterial activity against Escherichia coli strongly depended on the specific surface area of the powder; the activity increased with the increase of the surface area, irrespective of the shape and the crystallinity of their powders. In the case of Staphylococcus aureus, however, it was found that there was no difference in the activity according to the characteristic of the powders. The appearance of antibacterial activity was found to be due to the generation of H₂O₂ from the surface of ZnO powder.