In our previous studies by electron microscopy (EM) and two dimensional infrared spectroscopy of amyloid fibrils prepared in vitro from synthetic 40-residue β-amyloid (Aβ40) peptides, it was shown that the Aβ40 fibril structure and its molecular structure are not uniquely determined by amino acid sequence. Instead, the fibril structure is dependent upon the growth conditions. The molecular structures of β-amyloid fibrils that develop in Alzheimer's disease (AD) are therefore uncertain. In this study, fibrils extracted from brain tissue of AD model mice (three strains of aged transgenic mice with increased levels of human amyloid proteins, 3xTg-AD, J20Tg-AD and 5xFAD) were used to seed the growth of synthetic Aβ40 fibrils. Because amyloid fibril structures propagate themselves via seeded growth, the structures of seeded Aβ40 fibrils likely reflect structures in AD brain. Negatively stained EM images indicate that seeded fibrils tend to appear twisted like a ribbon, with periodic narrowing or nodes. The distances between nodes (the “internodal” distances) were relatively homogeneous distributions (∼145 nm) in the 3xTg-AD and J20Tg-AD mice. In 5xFAD, on the other hand, fibrils having short internodal distances (∼30 nm) were observed in addition to the distributions (∼145 nm) which were measured in two other strains of mice. The mass-per-length (MPL) evaluated from dark-field EM images indicates that the most prevalent numbers of filaments in fibrils are 3 and 4 but the dominant number of the filaments is dependent on the source of the fibril seeds. Overall, these results demonstrate that AD model mouse brain-derived fibrils have a distinct fibril structures, and the model most relevant to human AD has yet to be determined.