Alzheimer's disease (AD), an age-related neurodegenerative disorder, is the most common form of dementia, and the seventh-leading cause of death in the United States. Current treatments offer only symptomatic relief; thus, there is a great need for new treatments with disease-modifying potential. One pathological hallmark of AD is so-called senile plaques, mainly made up of β-sheet-rich assemblies of 40- or 42-residue amyloid β-peptides (Aβ). Hence, inhibition of Aβ aggregation is actively explored as an option to prevent or treat AD. Congo red (CR) has been widely used as a model antiamyloid agent to prevent Aβ aggregation. Herein, we report detailed morphological studies on the effect of CR as an antiamyloid agent, by circular dichroism spectroscopy, photo-induced cross-linking reactions, and atomic force microscopy. We also demonstrate the effect of CR on a preaggregated sample of Aβ(1-40). Our result suggests that Aβ(1-40) follows a different path for aggregation in the presence of CR.