In our previous paper, aminoethylated cotton cloth fixed with tannic acid was reported to have a high antibacterial activity against E. coli W3110 and S. aureus IFO13276. ln this paper, the results of the experiments carried out to investigate the mechanisms of the antibacterial action of the tannic acid-modified cotton cloth are described. Antibacterial activities of model compounds having different numbers of phenolic hydroxyl group were evaluated against E. coli W3110 and S. aureus IFO13276. lt was found that the value of minimum inhibitory concentration (MIC) of the model compound decreased with an increase in the number of phenolic hydroxyl group in a molecule. lt was also found that the model compound having a calboxyl group had a greater MIC than the corresponding model compound having no calboxyl group did. Thus, the antibacterial activities of phenolic compounds were ascribed to their phenolic hydroxyl groups. Aminoethylated cotton cloths fixed with model compounds having,different numbers of phenolic hydroxyl group and a calboxyl group showed antibacterial activities. ln this case, too, the activity increased with an increase in the number of hydroxyl group in the model compound fixed. As no distinct halo was observed in the culture-tests of the both bacteria the phenolic compounds were considered to have antibacterial activities in the state of being fixed on the cotton cloth.