The combined photocatalysis and ozonation (UV/Ag-TiO2+O3) process has been investigated for its potential use for ballast water treatment by usingDunaliella salina(D. salina) as an indicator microorganism. Inactivation curves were obtained, and the mechanism was studied. Compared to individual unit processes, the combined UV/Ag-TiO2+O3tests yield enhanced inactivation efficiency. Scanning electron microscopy (SEM) measurements showed that presence of O3during UV/Ag-TiO2exposure expedited the destruction of cell structure, which was further confirmed by the formation of malondialdehyde (MDA) and decrease in chlorophyll-a (chl-a) concentration. The results suggest that UV/Ag-TiO2+O3was efficient for inactivation ofD. salinain simulated ballast water, and the accelerated destruction of cell structure was an important reason for the enhanced inactivation efficiency.