Algae in water source causes many problems not only to the quality of treated water, but also to the operation of drinking water treatment processes. Therefore, it is essential to remove algae throughout water treatment process, especially during coagulation and preoxidation process. At coagulation experiment of a different coagulant species for removal algae, Microcystis showed that the removal efficiency was similar to alum, PSO-M, PASS and PAC coagulant. The coagulation efficiency of Stephanodiscus and Microcystis after prechlorination and preozonation treatment was increased the one that than use of only alum coagulant. Therefore circumstances of ozone treatment was prefitable removal efficiency of dissolved organic matter(DOC) and was few production of 1㎛ particles than chlorine treatment. The result of Algogenic Organic Matter (AOM) classification for Stephanodiscus and Microcystis, hydrophilic, humic and fulvic acid component showed each 59%, 26%, 15% and 38%, 36%, 26% order. The result after two day that only Microcystis input in Milli-Q water, DOC released from showed 1.16㎎/L. DOC released from the Microcystis after chlorine and ozone treatment increased to about 15∼62%. Also, According to the rapid sand filter tests after preoxidation processes of Stephanodiscus and Microcystis, the preoxidation process using ozone or chlorine lead to reduce the rate of headloss development and prevent from breakthrough of turbidity than only alum treatment.