Background: Ozone is considered one of the most effective disinfectants that can inactivate resistant pathogenic microorganisms in which conventional disinfectant such as chlorine and chlorine dioxide faildue to its strong biocidal oxidizing property. Objective: The study aimed at modeling of ozone as a disinfectant of indicators bacteria in the drinking water. Methods: This work involved a series of batch experiments with raw water, taken from the intake of El-Nozha Water Purification Plant, Alexandria governorate. The ozone doses applied in this study were 1.2, 2.2, 3.4, and 4.3 mg/l. The disinfected effluent was collected at 5, 10, 20, and 30 min. The indicator microorganisms HPC bacteria, total coliforms, faecal coliforms and Streptococcus faecalis were examined before and after the ozonation. Results: The optimum ozone dosage applied in raw water to achieve 90% reduction of the indicator microorganisms was 2.5 mg/l. The optimum contact time to achieve 90% reduction of the indicator microorganisms in raw water was 8 min. By applying of ozone as post disinfection on filtered water, the reduction percent of the indicator microorganisms were 100%. This study estimated that the ozone cost was 1.76 piasters/m³. Conclusion and Recommendations: Ozone as a primary disinfection must be applied on raw water to reduce the formation of THMs due to pre-chlorination of raw water at present, in addition to effective killing power of ozone onto microorganisms that will improve water quality.