"Oxidative stress" is defined as a disturbance in the oxidant-antioxidant balance in favor of the former, and is involved in a numerous number of diseases. We identified the structure of oxidatively modified peroxiredoxin (Prx) and DJ-1 under the oxidative stress condition using mass spectrometry. The ratios of the oxidatively modified Prx 2 and Prx 6 to the reduced form in erythrocytes of Alzheimer's disease (AD) patients were elevated compared to those of the healthy control, respectively. These modified proteins are promising candidate markers for diagnosis of AD.