Objective With the inclusion of cooperative effects, we investigate the β-propensity of amyloidogenic peptides (and proteins) on amyloid nucleation. Methods Based on Monte Carlo simulations using a coarse-grained model for amyloidogenic peptides containing two states, i.e., a soluble state and a β-prone state (denoted by s and β state, respectively), we investigate how β-propensity of peptides affect amyloid nucleation, under the considerations of two classes of cooperative effects – the existence of surrounding peptides “inhibiting” and “promoting”, respectively, the peptide’s s-to-β conformational conversion. Results In the “inhibiting” case, amyloid nucleation occurs only within a certain interval of the peptide concentration, and occurs inside the oligomers with certain sizes. Besides, we observed the coexistence of on-pathway and off-pathway oligomers. In the “promoting” case, we observed, for the first time to our knowledge, amyloid nucleation at a peptide concentration as low as ~4 μM, which is the lowest nucleation concentration found in experiments (in vitro). Conclusions In this study, we develop a more realistic model for amyloidogenic peptides (and proteins) by introducing surrounding-monomer-dependent β-propensity of peptides. Using this model, we observe some intriguing phenomena not reported in previous simulations. Our findings not only improve current understandings of the mechanism of amyloid formation, but also provide useful insights into the therapeutic strategies for curing neurodegenerative diseases.