In the present study, the characteristics of the combined ultraviolet (UV) and ozone disinfection process were investigated from kinetic and mechanistic viewpoints employing Escherichia coli (E. coli) as an indicator microorganism. Compared to individual unit processes, the combined UV/O3 tests produced excess hydroxyl radicals (HO•) and yielded synergistic inactivation of E. coli in the initial phase of reaction. The presence of O3 during UV exposure caused the destruction of cell structure, and then repressed bacteria regrowth after treatment. Moreover, the formation of malondialdehyde (MDA) showed that the improved generation of intermediate HO• via ozone photolysis accelerated the decomposition of bacterial cell surfaces, which was further confirmed by the leakage of intracellular potassium ions (K+). The results suggested that the synergistic bactericidal effect of combined UV/O3 owed mainly to the enhanced destruction of bacterial cell structure.