The effects of particlesize and heating temperature of magnesium oxide (MgO), calcium oxide (CaO), and zinc oxide (ZnO) powders on the antibacterial activities of their slurries were studied by the conductance method. An increase in particle size of the powders reduced their antibacterial activity. The variation of activity against Staphylococcus aureus was smaller than that against Escherichia coli, Salmonella typhimurium, and Bacillus subtilis. This difference depended on the shape of the bacteria, but not their character, such as the structure of the cell walls. The antibacterial activity of these powder slurries decreased with an increase in heating temperature of the powders. For the ZnO powder slurry, the decrease in antibacterial activity were much larger than those of the MgO and CaO powder slurries. It was suggested that these decreases in the antibacterial activity was concerned with stabilization of the surface of the powders heated at high temperature.