Accumulation of unfolded protein within the endoplasmic reticulum (ER) attenuates mRNA translation through PERK-mediated phosphorylation of eukaryotic initiation factor 2 on Ser51 of the α subunit (eIF2α). To elucidate the role of eIF2α phosphorylation, we engineered mice for conditional expression of homozygous Ser51Ala mutant eIF2α. The absence of eIF2α phosphorylation in β cells caused a severe diabetic phenotype due to heightened and unregulated proinsulin translation; defective intracellular trafficking of ER cargo proteins; increased oxidative damage; reduced expression of stress response and β-cell-specific genes; and apoptosis. However, glucose intolerance and β cell death in these mice were attenuated by a diet containing antioxidant. We conclude that phosphorylation of eIF2α coordinately attenuates mRNA translation, prevents oxidative stress, and optimizes ER protein folding to support insulin production. The finding that increased proinsulin synthesis causes oxidative damage in β cells may reflect events in the β cell failure associated with insulin resistance in type 2 diabetes.