In Alzheimer's disease (AD), amyloid β (Aβ) protein plays a detrimental role in neuronal injury and death. Recent in vitro and in vivo studies suggest that soluble oligomers of the Aβ peptide are neurotoxic. Structural properties of the oligomeric assembly, however, are largely unknown. Our present investigation established that the 40-residue-long Aβ peptide (Aβ40) became more helical, ordered, and compact in the oligomeric state, and both the helical and β-sheet components were found to increase significantly in the early event of oligomerization. The band-selective two-dimensional NMR analysis suggested that majority of the residues from sequence 12 to 22 gained a higher-ordered secondary structure in the oligomeric condition. The presence of a significant amount of helical conformation was confirmed by Raman bands at 1650 and 1336 cm-1. Other residues remained mostly in the extended polyproline II (PPII) and less compact β-conformation space. In the event of maturation of the oligomers into an amyloid fiber, both the helical content and the PPII-like structural components declined and ∼72% residues attained a compact β-sheet structure. Interestingly, however, some residues remained in the collagen triple helix/extended 2.51-helix conformation as evidenced by the amide III Raman signature band at 1272 cm-1. Molecular dynamics analysis using an optimized potential for liquid simulation force field with the peptide monomer indicated that some of the residues may have preferences for helical conformation and this possibly contributed in the event of oligomer formation, which eventually became a β-sheet-rich amyloid fiber.