Recent experimental studies have shown that amyloid fibril formed by aggregation of beta peptide exhibits excellent mechanical properties comparable to other protein materials such as actin filaments and microtubules. These excellent mechanical properties of amyloid fibrils are related to their functional role in disease expression. This indicates the necessity of understanding how an amyloid fibril achieves the remarkable mechanical properties through self-aggregation with structural hierarchy. However, the structure-property-function relationship still remains elusive. In this work, the mechanical properties of human islet amyloid polypeptide (hIAPP) are studied with respect to its structural hierarchies and structural shapes by coarse-grained normal mode analysis. The simulation shows that hIAPP fibril can achieve the excellent bending rigidity via specific aggregation patterns such as antiparallel stacking of beta peptides. Moreover, the length-dependent mechanical properties of amyloids are found. This length-dependent property has been elucidated from a Timoshenko beam model that takes into account the shear effect on the bending of amyloids. In summary, the study sheds light on the importance of not only the molecular architecture, which encodes the mechanical properties of the fibril, but also the shear effect on the mechanical (bending) behavior of the fibril.