The number of patients with Alzheimer's disease (AD) and type 2 diabetes (T2D) is increasing rapidly, and thus more research has been focused on the relationship between these two age-related chronic diseases. According to the amyloid hypothesis, prevention of the aggregation of amyloid beta (A beta) and human islet amyloid polypeptide (hIAPP) is a promising strategy for AD and T2D. In this study, thioflavin-T assay and transmission electron microscopy were performed to evaluate the inhibitory effect of three phenylpropanoids isolated from Lycopus lucidus-schizotenuin A and lycopic acids A and B-on both A beta and hIAPP fibrillization. All tested compounds exhibited similarly strong inhibitory activity toward amyloid aggregation. These results suggested that catechol moieties play important roles in the inhibition of amyloid plaque formation.