In this study, the validity of TiO2-based photocatalysts anti-algal effects was examined with Chlorella vulgaris. Cu-modified TiO2 (without N dopants) showed clear anti-algal effect under white LED light. N-doped TiO2 (both with and without Cu modification) and Cu-modified TiO2 (without N dopants) showed clear anti-algal effects when UV light was used together with white LED light. S-doped TiO2 had no anti-algal effects and even promoted the growth of algae. The degree of the anti-algal effect differed depending on the dopants, surface modifications, and irradiation light. Photocatalysts create several active spices during their photocatalytic processes. Of these, the effects of superoxide radical (O (2) (center dot-) ) and hydrogen peroxide (H2O2) on algae growth were examined. Photocatalysts with high anti-algal effect produced hydrogen peroxides effectively, while there was no correlation between productivity of superoxide radical and anti-algal effects. Thus, the ability to produce hydrogen peroxide is a plausible factor for determining the efficiency of the anti-algal effect of a photocatalysts.