Water quality that used by Yogyakarta people for daily activity surprisingly contain big numbers of bacterias such as e-colli or other bacteria. It shows that the water need a sterilization before it is used by people to prevent desease that might appears during the usage of that bacteriacontaminated water. One of the ways to sterilize the water is by doing ozonisation. The Ozonizerbased ATMega 8353 is an equipment that has been designed by the writer to sterilize or reduce the number of bacterias on the water. The writer used quantitative methods to observe the function of the equipment by took three samples of water, those are; river water, well water, and tub water. After been sterilizied for the same duration of nine minutes by The Ozonizer-based ATMega 8353, it is found that the number of bacterias on river water reduced from 1311 colonies into 636 colonies. Then for well water bacterias, they had reduced from 954 colonies into 354 colonies. Meanwhile, for tub water bacterias, they had reduced from 1041 colonies into 269 colonies. In conclusion, this equipment, The Ozonizer-based ATMega 8353, successfuly functions to reduce the number of bacterias on the water.