The effect of UV on the mortality rate of toxic dinoflagellate Amphidinium carterae causing a red tide in the coastal area of south Korea was investigated in the batch and continuous-scale reactor equipped with ultraviolet irradiation-apparatus. Degussa P25 titanium oxide, a photocatalyst proved to be effective for the mortality of Amphidinium carterae supplied with photocatalyst and UV radiation were greater than 95% in 2 minutes of UV radiation and the rate were higher than that by UV-radiation without titanium dioxide in the batch and continuous-flow scale reactor. The mortality time of Amphidinium carterae increased with the cell density under UV-illumination in the batch scale reactor. The mortality rate in the density of 5.0×104 cells/mL at the same experimental condition was more than 90% in 4 minutes in the continuous flow scale reactor. The percentage of 99.9±0.1% of Amphidinium carterae in the density of 5.0×104 cells/mL was died in 20 minutes when the phytoplankton was illuminated with UV-radiation without photocatalyst.