Aggregation of β-amyloid (Aβ) has been considered as an important factor leading to Alzheimer’s disease (AD). Previous studies have focused on the full-length unmodified Aβ_1-40 and Aβ_1-42. In recent years, it was found that a variety of truncated and modified Aβ species co-exist in AD patients’ brain. These Aβ species contributed to the progression of AD and should not be overlooked. For example, the emergence of pyroglutamated Aβ and phosphorylated Aβ is considered as symptoms of AD. And the most abundant Aβ species in AD patients’ brain should be Aβ_4-40/42, which has similar aggregation properties and toxicity with Aβ_1-40/42. Due to the elevated oxidative stress in AD patients’ brain, tyrosine nitration, dimerization and methionine oxidation were also identified in Aβ sequence, resulting in different properties. We review here the production, structure and toxicity of different Aβ species and summarize their possible roles in AD pathology.