Unlike previous in vitro measurements where Amyloid beta (A beta) aggregation was studied in bulk solutions, we detect the structure change of the A beta aggregate on the surface of a wireless quartz-crystal-microbalance biosensor, which resembles more closely the aggregation process on the cell membrane. Using a 58 MHz quartz crystal, we monitored changes in the viscoelastic properties of the aggregate formed on the quartz surface from monomers to oligomers and then to fibrils, involving up to the 7th overtone mode (406 MHz). With atomic-force microscopy observations, we found a significant stiffness increase as well as thinning of the protein layer during the structure change from oligomer to fibrils at 20 h, which indicates that the stiffness of the fibril is much higher. Viscoelasticity can provide a significant index of fibrillation and can be useful for evaluating inhibitory medicines in drug development.