The common neurodegenerative disorder known as Alzheimer’s disease is characterized by cerebral neuritic plaques of amyloid β (Aβ) peptide. Plaque formation is related to the highly aggregative property of this peptide, because it polymerizes to form insoluble plaques or fibrils causing neurotoxicity. Here, we expressed Aβ peptide as a new causing agent to endoplasmic reticulum (ER) stress to study ER stress occurred in plant. When the dimer of Aβ1–42 peptide was expressed in maturing seed under the control of the 2.3-kb glutelin GluB-1 promoter containing its signal peptide, a maximum of about 8 μg peptide per grain accumulated and was deposited at the periphery of distorted ER-derived PB-I protein bodies. Synthesis of Aβ peptide in the ER lumen severely inhibited the synthesis and deposition of seed storage proteins, resulting in the generation of many small and abnormally appearing PB bodies. This ultrastructural change was accounted for by ER stress leading to the accumulation of aggregated Aβ peptide in the ER lumen and a coordinated increase in ER-resident molecular chaperones such as BiPs and PDIs in Aβ-expressing plants. Microarray analysis also confirmed that expression of several BiPs, PDIs and OsbZIP60 containing putative transmembrane domains was affected by the ER stress response. Aβ-expressing transgenic rice kernels exhibited an opaque and shrunken phenotype. When grain phenotype and expression levels were compared among transgenic rice grains expressing several different recombinant peptides, such detrimental effects on grain phenotype were correlated with the expressed peptide causing ER stress rather than expression levels.