Under hot weather condition, air conditioner is an important household device which control temperature and humidity of a closed chamber. A filtration in the air conditioner has functions in reducing dust and microorganisms of inlet airflow. However, humidity in conditioner system may enhance microbial growth inside the device and re-distribution in flowing out air. Furthermore, airborne pathogens are being concern because of epidermic crisis. In this research work, we investigated the impact of ozone, an air cleaning system, on reducing microorganism quality and quantity of airflow out of the conditioner. The ozone generating system is constructed and cooperated over the airflow channel. This ozone system is opened for fifteen minutes before placing enrich media at five different positions of pre-conditioning room. The medium plates were opened for fifteen minutes at 0, 3, 6, 12 and 24 hours after generating ozone. Collecting plates were incubated at room temperature and counted the microorganism colony for four days. The result showed that ozone generator cooperated over airflow out of the conditioner evidently presented a smaller number of microorganisms (bacteria, yeast, and fungi) in compared to non-cooperated system did. Therefore, the cooperated ozone treatment is effectiveness in eliminating microorganisms in the air conditioning room.