© The Korean Society of Plant Pathology. To investigate the difference in the disinfectant efficiency of ozone microbubbles (O3MB) and ozone millibub-bles (O3MMB), the morphological change of the treated Fusarium oxysporum f. sp. melonis spores was observed with scanning and transmission electron microscopies (SEM and TEM). The disinfectant efficiency of O3MB on F. oxysporum f. sp. melonis spores was greater than that of O3MMB. On observation with SEM, it was revealed that morphological change of F. oxysporum f. sp. melonis spores was caused by O3MB and O3MMB, and damage to the spore surfaces by O3MB occurred sooner than that by O3 MMB. On observation with TEM, it was furthermore confirmed that F. oxysporum f. sp. melonis spores treated with O3MB induced wavy deformation of cell membrane and the intracellular change different from that with O3MMB. Therefore, the greater disinfection efficiency of O3MB was suggested to be caused due to the function of the MB in addition to the oxidative power of O3.