Oxidative stress is related to the pathogenesis of Alzheimer's disease (AD) characterized by progressive memory impairment. Soluble amyloid-β (Aβ) oligomers cause cognitive loss and synaptic dysfunction rather than senile plaques in AD. The decline of the antioxidant status is associated with dementia in AD patients, especially low levels of vitamin C. Our group previously reported a relationship between anti-aging and supplementation of vitamin C derivatives. Here we report that vitamin C mitigated Aβ oligomer formation and behavioral decline in an AD mouse model treated with a vitamin C solution for 6 months. The attenuation of Aβ oligomerization was accompanied with a marked decrease in brain oxidative damage and in the ratio of soluble Aβ₄₂ to Aβ₄₀, a typical indicator of AD progression. Furthermore, the intake of vitamin C restored the declined synaptophysin and the phosphorylation of tau at Ser396. On the other hand, brain plaque deposition was not altered by the dietary intake of vitamin C. These results support that vitamin C is a useful functional nutrient for the prevention of AD.