Amyloid beta, A beta(1-42), is a component of senile plaques present in the brain of Alzheimer's disease patients and one of the main suspects responsible for pathological consequences of the disease. Herein, we directly visualize the A beta activity toward a brain-like model membrane and demonstrate that this activity strongly depends on the A beta oligomer size. PeakForce quantitative nanomechanical mapping mode of atomic force microscopy imaging revealed that the interaction of large-size (LS) A beta oligomers, corresponding to high-molecular-weight A beta oligomers, with the brain total lipid extract (BTLE) membrane resulted in accelerated A beta fibrillogenesis on the membrane surface. Importantly, the fibrillogenesis did not affect integrity of the membrane. In contrast, small-size (SS) A beta oligomers, corresponding to low-molecular-weight A beta oligomers, created pores and then disintegrated the BTLE membrane. Both forms of the A beta oligomers changed nanomechanical properties of the membrane by decreasing its Young's modulus by similar to 45%. Our results demonstrated that both forms of A beta oligomers induce the neurotoxic effect on the brain cells but their action toward the membrane differs significantly.