Oligomer species of amyloid beta (A beta) peptides are intensively investigated because of their relevance to Alzheimer's disease (AD), and a stable oligomer will be a cause of AD. In this article, we investigate the structural stability of two representative A beta(1-40) oligomers, which are with and without the beta-sheet structure, denoted by beta and non-beta oligomers, respectively, using optimized ultrasonic irradiation (OUI). Recent studies reveal that OUI significantly accelerates the fibril formation in A beta(1-40) monomers; it is capable of transforming any unstable oligomers into fibrils (the dead-end products) in a short time. First, we find that beta oligomers can be produced under high-speed stirring agitation; their beta-sheet structures are evaluated by the circular-dichroism spectrum measurement, by the immunoassay using the fibril-specific OC antibody, and by the seeding experiment, showing identical characteristics to those formed in previous reports. Second, we form non-beta oligomers in a high-concentration NaCl solution and confirm that they include no beta-sheet structure, and they are recognized by the oligomer-specific All antibody. Furthermore, we confirm the neurotoxicity of the two types of oligomers using the neural tissue derived from mouse embryonic stem cells. We apply the OUI agitation to the fi and non-beta oligomers. The non-beta oligomers are transformed into the fibrils, indicating that they are intermediate species in the fibrillation pathway. However, the fi oligomers are surprisingly unaffected by OUI, indicating their high thermodynamic stability. We conclude that the beta oligomers should be the independent dead-end products of another pathway, different from the fibrillation pathway.