The results show that ozone concentration determination using ultraviolet spectrophotometry (UV-2450) at 258 nm is easier than using indigo method at 600 nm. A strong linear relationship was found between purge time and O3 concentration in deionized water. Ozone concentration can be predicted in deionized water. A higher O3 flow rate or lower temperature led to a higher O3 concentration. Ozone concentration was stable in 60 min, so that ozone self-decomposition could be ignored at ozone concentrations below 0.4 mg L-1. A higher temperature led to a higher inactivation efficiency and rate, and that a lower temperature led to a lower ozone decay rate and inactivation efficiency even if ozone solubility increased when temperature decreased. The fastest inactivation rate occurred before c0t = 165 μg L-1 s, but the inactivation rate decreased after c0t = 165 μg L-1 s with tail phenomena. The tail phenomena were clearly observed and may be caused by oxidization of lipopolysaccharides (LPS), cell membrane, etc. The activation energy Ea = 55,404 ± 0.3 J mol-1 were obtained for Legionella inactivation with ozone in deionized water. Ozone maximum decay rate was positively proportional to COD concentration. COD impacted on ozone concentration seriously. Higher COD concentration resulted in higher ozone decay rate. COD could result in ozone concentration decrement rapidly to a steady value in 5 s. Higher initial ozone concentration resulted in higher germ inactivation rate. Higher initial COD concentration resulted in lower Legionella inactivation efficiency. COD was easier to react with ozone than Legionella. The relationship among the initial COD concentrations COD0, the initial O3 concentration c0, and the O3 contact time t necessary for a 99.999% reduction of Legionella in wastewater can be expressed in some equations. O3 disinfection time t necessary for a 99.999% reduction of Legionella can be predicted by Eqs. (10) and (11). Graphical abstract ᅟ.