Although the antibacterial activity of ceramics has been reported in many literatures, there are none that report on the significant relation between the crystallite size and antibacterial activity. The influence of the crystallite size on antibacterial activity of MgO and ZnO powder samples was investigated in the present study. The crystallinity of powder samples was determined by X-ray diffraction measurement From Williamson-Hall plots calculated from obtained diffraction peaks, it was found that differences in crystallinity were dependent on the crystallite size. Antibacterial activity of the powder samples was examined by the colony count method, using Escherichia coil and Staphylococcus aureus. The powder samples with a small crystallite size showed greater antibacterial activity than those with a large crystallite size. We found that the crystallite size affects the antibacterial activity of MgO and ZnO powder samples. (C) 2011 Elsevier Ltd. All rights reserved.