In this study, disinfection effect of coliform group treated with ozone was analyzed for the effluent discharging from activated sludge treatment process in hospital wastewater treatment plant. Based on these results, it aims to get basic data to establish the disinfection facility in hospital wastewater treatment, suggesting affecting factors according to hydraulic retention time of the activated sludge process and design basic factors necessary to ozone disinfection. Removal efficiencies of coliform group for the effluent according to HRT such as 4hr, 8hr, 12hr were analyzed using a lab-scale activated sludge process, respectively. To decay the microorganism regulates the CT value multiplied disinfectant concentration by contact time. CT value is highly correlated with a removal efficiency of coliform group and according to increasing an injected ozone concentration, the CT value was increased.. When HRT was 4, 8, 12 hours , CT value takes place at minimum condition 50, 40, 334 mg/L-min, respectively.