Tau protein is essential to assembly of microtubules, which mainly consist of two types of tubulin. Hyperphosphorylation of tau protein abolishes its ability to bind tubulin and promote microtubule assembly. When it is released from tubulin, the phosphorylated tau protein aggregates into paired helical filaments, which are characterized as the neuropathological hallmarks of Alzheimer's disease. The mechanism of aggregation of tau protein is still unsolved, though great efforts have been made to elucidate it. Recently, several peptidyl-prolyl isomerases were suggested to interact with tau protein and rescue it from aggregation. Here we investigated how these isomerases interacted with aggregate-prone peptides extracted tau protein by fluorescence spectroscopy, dynamic light scattering measurement, and ELISA. As the result, each isomerase site-specifically interacted with the peptides and suppressed their aggregation by the peptidyl-prolyl isomerase activity.