Background: Amyloid β-protein (Aβ) and α-synuclein (αS) are the primary components of amyloid plaques and Lewy bodies (LBs), respectively. Previous in vitro and in vivo studies have suggested that interactions between Aβ and αS are involved in the pathogenesis of Alzheimer's disease (AD) and LB diseases (LBD). However, the cross-seeding effects of their aggregates on their aggregation pathways are not completely clear. Methods: To investigate the cross-seeding effects of Aβ and αS, we examined how sonicated fibrils or cross-linked oligomers of Aβ40, Aβ42, and αS affected their aggregation pathways using thioflavin T(S) assay and electron microscopy. Results: Fibrils and oligomers of Aβ40, Aβ42, and αS acted as seeds, and affected the aggregation pathways within and among species. The seeding effects of αS fibrils were higher than those of Aβ40 and Aβ42 fibrils in the Aβ40 and Aβ42 aggregation pathways, respectively. Their oligomers were less efficient than fibrils. Conclusion: The fibrils and oligomers of Aβ40, Aβ42, and αS functioned as seeds and promoted each other's aggregation pathways in vitro, though the oligomers were less efficient than fibrils. The cross-seeding effects observed in this study may provide new insights into the molecular mechanisms that underlie the interactions between AD and LBD pathogenesis.