Antibacterial activities of metallic oxide (ZnO, MgO and CaO) powders against Staphylococcus aureus and Escherichia coli were quantitatively evaluated by measuring the change in electrical conductivity of the growth medium caused by bacterial metabolism (conductimetric assay). The obtained conductivity curves were analyzed using the growth inhibition kinetic model proposed by Takahashi for calorimetric evaluation, and the metallic oxides were determined for the antibacterial efficacy and kinetic parameters. The parameters provide some useful indicators for antimicrobial agents, such as the dependence of antibacterial activity on agent concentration, and the affinity between the agent and the bacterial cells. CaO was the most effective, followed by MgO and ZnO, against E. coli. On the other hand, ZnO was the most effective for S. aureus and was suggested to have a strong affinity to the cells of S. aureus.