In the amyloid plaques of Alzheimer's disease (AD) patients, a large number of N-terminal-truncated amyloid β (Aβ) peptides such as Aβ11-40 have been identified in addition to the full-length Aβ peptides. However, little is known about the roles of the N-terminal-truncated peptides in AD pathological process. Herein, seeding and cross-seeding aggregations of Aβ40 and its N-terminal-truncated Aβ11-40 were investigated in the solution and on the surfaces of chips with immobilized seeds by extensive biophysical and biological analyses. The results showed that Aβ40 and Aβ11-40 aggregates could seed both homologous and heterologous aggregations of the two monomers. However, the capability and characteristics of the seeding (homologous aggregation) and cross-seeding (heterologous aggregation) were significantly different. Aβ40 seeds showed stronger acceleration effects on the aggregations than Aβ11-40 seeds and induced β-sheet-rich fibrous aggregates of similar cytotoxicities for the two monomers. This indicates that Aβ40 and Aβ11-40 had similar aggregation pathways in the seeding and cross-seeding on Aβ40 seeds. By contrast, Aβ11-40 seeds led to different aggregation pathways of Aβ40 and Aβ11-40. Pure Aβ11-40 aggregates had higher toxicity than Aβ40 aggregates, and as seeds, Aβ11-40 seeds induced Aβ40 to form aggregates of higher cytotoxicity. However, homologous Aβ11-40 aggregates induced by Aβ11-40 seeds showed lower cytotoxicity than pure Aβ11-40 aggregates. The results suggest that Aβ11-40 plays an important role in the pathological process of AD.