The mechanism of the inactivation of Lactobacillus casei phage PL-1 suspended in a phosphate buffer by black-light (BL) -catalytic titanium dioxide (TiO2) thin film was studied. Generation of both superoxide anions (O-2(-)) and hydroxyl radicals (. OH) was confirmed in the aqueous medium in which TiO2 film was settled with BL irradiation under gentle shaking. With BL-irradiation alone without TiO2 film, only O-2(-) was generated to some extent. The genome DNA inside the phage particles was found to be fragmented by the treatment of PL-1 phages with BL-catalytic TiO2 film. The phage inactivation by BL-catalytic TiO2 film was inhibited by the addition of albumin in a concentration-dependent manner. BL-catalytic TiO2 film was considered to cause primarily the damage to the capsid protein through the generation of active oxygen species such as . OH, followed by damage to the genome DNA inside the phage particles.