Aggregations of proteins are in many cases associated with neurodegenerative diseases such as Alzheimer's (AD). Small compounds capable of inhibiting protein aggregation are expected to be useful for not only in the treatment of disease but also in probing the structures of aggregated proteins. In previous studies using phage display, we found that arginine-rich short peptides consisting of four or seven amino acids bound to soluble 42-residue amyloid β (Aβ42) and inhibited globulomer (37/48 kDa oligomer) formation. In the present study, we searched for arginine-containing small molecules using the SciFinder searching service and tested their inhibitory activities against Aβ42 aggregation, by sodium dodecyl sulfate (SDS)-PAGE and thioflavine T binding assay. Commercially available Arg-Arg-7-amino-4-trifluoromethylcoumarin was found to exhibit remarkable inhibitory activities to the formation of the globulomer and the fibril of Aβ42. This chimera-type tri-peptide is expected to serve as the seed molecule of a potent inhibitor of the Aβ aggregation process.