This study uses blue LED light (lambda(max) = 475 nm) activated TiO2/Fe3O4 particles to evaluate the particles' photocatalytic activity efficiency and bactericidal effects in seawater of variable salinities. Different TiO2 to Fe3O4 mole ratios have been synthesized using sol-gel method. The synthesized particles contain mainly anatase TiO2, Fe3O4 and FeTiO3. The study has identified TiO2/Fe3O4's bactericidal effect to marine fish pathogen (Photobacterium damselae subsp. piscicida BCRC17065) in seawater. The SEM photo reveals the surface destruction in bacteria incubated with blue LED irradiated TiO2/Fe3O4. The result of this study indicates that 1) TiO2/Fe3O4 acquires photocatalytic activities in both the freshwater and the seawater via blue LED irradiation, 2) higher photocatalytic activities appear in solutions of higher TiO2/Fe3O4 mole ratio, and 3) photocatalytic activity decreases as salinity increases. These results suggest that the energy saving blue LED light is a feasible light source to activate TiO2/Fe3O4 photocatalytic activities in both freshwater and seawater. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.