Ap amyloid protein is representative agent, which is involved in neuro-degenerative diseases. Due to the undegrading characteristics under physiological conditions, understanding the structural characteristics of Aβ amyloid protein in detail is crucial. Many efforts have made on the lowering the structural stabilities of Aβ amyloid protein by decreasing the aromatic residues characteristic and hydrophobic effect. However, there lacks an understanding of Aβ amyloid pair structures in detail. In this work, we provide the structural characteristics of Aβ amyloid pair structures by selective leucine residue mutation on phenylalanine residue (F20L). We also considered the polymorphic feature of F20L Aβ amyloid pair based on NN, CC and NC compositions. Furthermore, we found the structural difference of oligomeric and fibrillar NC F20L Aβ amyloid pair structures in detail.