Ther effect of UV on the mortality rate of toxic dinoflagellate Amphidinium Carterae causing a red tide in the coastal area of korea was investigated in the batch and continuous-scale reactor equipped with ultraviolet irradiation-apparatus. Degussa P(sub)25 titanium oxide, a photocatalyst proved to be effective of 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 $5.0$ imes$10^4$</TEX> cell/mL at the same experimental condition was more than 90% in 4 minutes in the continuous flow scale reactor. The percentage of 99.9$pm$0.1% of Amphidinium Carterae in the density of $0.5$ imes$10^4$</TEX> cells/mL was died in 20 minutes when the phytoplankton was illuminated with UV-radiation without photocatalyst.