This study was conducted through jar-test and pilot-plant test to research coagulation effect of pre-ozonation on the contaminated water by organic matter (TOC), inorganic matter (kaolin) and algae specially diatom (chlorophyll-a). Pre-ozonation had not coagulation effect on the kaolin water further more acted as coagulation deteriorator on the organic matter water which imvolved humic acid more than 2㎎/ℓ as total organic carbon (TOC). But it had good coagulation effect on the nutriented water more than 40μ/ℓ as chlorophyll-a. Therefore pre-ozonation utility is desired for treating nutriented source water. We had added ozone to the netriented raw water increasingly 1㎎/ℓ from 1 to 5㎎/ℓ, About l∼3㎎/ℓ ozone treated water was appeared lower residual turbidity than of 5㎎/ℓ ozone contacted water and untreated water after jar-test. Compared residual turbidity of l㎎/ℓ ozone treated water to 3㎎/ℓ, we did not find significant difference of residual turbidity between two jar-test data. So we proposed 1㎎/ℓ as optimum pre-ozone dose for nutriented water. And pre-ozonation process had better coagulation effect to the higher chlorophyll-a concentration water. Authors also found out that pre-ozonation could save coagulant about 25%.