Evaluated here are the inhibitory effects on blue-green algae (Microcystis aeruginosa) produced by nine plant-producing phenols (caffeic, p-coumaric, ferulic, protocatechuic, sinapic, syringic, and vanillic acids, catechol, and hydroquinone), two plant-produced acids (quinic and shikimic acid), phenol, resorcinol, hydroxy hydroquinone, and phloroglucinol. Algal assays confirmed growth inhibition of M. aeruginosa by polyphenols, i.e., caffeic/protocatechuic acid, catechol, hydroquinone, hydroxy hydroquinone, and phloroglucinol, and by phenols containing methoxy groups, i.e., vanillic, sinapic, and syringic acids. Accordingly, this indicates good feasibility for controlling growth of M. aeruginosa using such plant-producing polyphenols and/or phenols as additives. A comparison of the inhibitory effects of the polyphenols showed that those induced by polyphenols in which phenolic hydroxy groups bound a benzene ring at ortho- and/or para-positions to another phenolic hydroxy group are stronger than the effects induced by polyphenols in which phenolic hydroxy groups are at only meta-positions. Experiments showed that the only polyphenols demonstrating significant growth inhibition of M. aeruginosa were autoxidized. These results suggest that autoxidation of the polyphenols induces inhibitory effects by producing polyphenol-autoxidized products such as radicals.