The antibacterial activity of zinc oxide nanoparticles prepared by sonication method was evaluated in this paper. Effect of calcination conditions on the size and antibacterial activity of zinc oxide nanoparticles was investigated. Zinc oxide nanoparticles were characterised for purity (Thermo Gravimetric Analyzer), crystallinity and crystal size (XRD), particle size and morphology (TEM), etc. Results showed that the smallest size 8 nm could be obtained. The lethal experiments of nanocrystalline zinc oxide were evaluated on Lactobacillus plantarum. All L. Plantarum were killed after 24-h exposure at 1000 ppm. With the increase in calcination time, the lethal effect of ZnO nanoparticles was increased after 6-h exposure at 100 or 1000 ppm.