Conventional coagulation is still the main treatment process for algae removal in water treatment. The coagulation efficiency can be significantly improved by the preoxidation of algae-containing water. Jar test was conducted to determine the optimal condition for the removal of diatoms, especially Cyclotella sp. by preoxidation and the subsequent coagulation. The effects of various concentration of PAC (Polyaluminum chloride) on coagulation with and without preoxidation using chlorine or potassium permanganate at different pHs (7.7 and 9.0) were evaluated. At pH 7.7, preoxidation with 2ppm $Cl_2$ followed by coagulation with 7.5 ppm PAC coagulant could reduce Cyclotella sp. concentration by 86%. At pH 9.0, preoxidation with 1 mg $KMnO_4/L$ followed by coagulation with 12.5 ppm PAC coagulant reduced Cyclotella sp. concentration by 85%. Non-linear regression was applied to determine the optimal condition. At pH 7.7 and 9.0, R was over 0.9, respectively. The pH of algal blooming water is over 9.0. Algae (diatom; Cyelotella sp.) can be controlled in the following ways: preoxidation with 1 mg $KMnO_4/L$ followed by coagulation with 12.5 ppm PAC coagulant can remove 80% algae from water. If water pH is adjusted to 7.7, it was expected that less amount of coagulant (7.5 or 10 mg PAC /L) after preoxidation ($Cl_2$ 2 ppm or $KMnO_4$ 0.33, 1 ppm) would be needed to achieve similar level of algae removal. The oxidation with 0.33ppm $KMnO_4$ followed by coagulation with 7.5 ppm PAC coagulant was preferable due to cost-effectiveness of treatment condition and color problem after treatment.