The sole ozone used in cooling water treatment has raised some critical questions concerned with what happens chemically, that may describe why it can control corrosion, scale and microorganism. The pilot cooling tower (PCT) test presented the effects of ozone as a sole cooling water treatment. Except the higher pH and alkalinity in the PCT test, proper residual ozone concentration produced an oxidative film (protective film) on the metal surface of carbon steel, brass and type 316 stainless steel, the corrosion rate may also be decreased. Ozone was still unpredictable for scale control except for the existence of organic matter. The excellent control of fouling was also attained, that could be determined by the presence of ozone. The strong biocide properties of ozone resulted in excellent biological control. A mechanism is proposed to explain the results.