Understanding the structure of amyloid-β (Aβ) aggregates is a key step towards elucidating the pathology of Alzheimer’s disease. In this work, three fragments of the Aβ1–42 protein, Aβ1–25 (DAEFRHDSGYEVHHQKLVFFAEDVG), Aβ25–35 (GSNKGAIIGLM), and Aβ33–42 (GLMVGGVVIA), were synthesized, and their aggregated structures were examined by linear infrared spectroscopy in the amide-I (mainly the CO stretching) region. The structures of the formed aggregates were found to be both sequence and pH dependent. The results suggest that instead of forming matured fibrils, as in the case of full-length Aβ1–42, both Aβ1–25 and Aβ33–42 form a mixture of threadlike β-sheet fibril, soluble β-sheet oligomer, and random coil structures. The β-sheet conformations were found to be mainly antiparallel for the former and both parallel and antiparallel for the latter. However, the Aβ25–35 fragment was found to form assembled fibrils containing predominantly parallel β-sheets. The conformation and morphology of the aggregates were also confirmed by circular dichroism measurements and transmission electron microscopy. Factors influencing the structures of the aggregates formed by the Aβ fragments were discussed.