Objective: Prevalence of diabetes mellitus (DM) had rapidly increased in the elderly. Because of high comorbidity in the elderly, early diagnosis and treatment of diabetes mellitus is very importance to reduce the morbidity and mortality by diabetic complication. In this study, we evaluated and compared the diagnostic rate of diabetes mellitus in the elderly using fasting plasma glucose (FPG), 2-hour plasma glucose (2h PG) after 75 g oral glucose tolerance test (OGTT), and glycosylated hemoglobin (HbA1C) levels. Methods: We retrospectively analyzed the medical records of 271 participants who have all results of FPG, 2h PG after 75 g OGTT and HbA1C. Participants divided in two groups according to age (young DM group, 59 years or younger vs old DM group, 60 years or older). Results: In old DM group, cut-off values for most accurate diagnosis of DM were 6.4% of HbA1C, 117 mg/dl of FPG, and 195 mg/dl of 2h PG after 75 g OGTT. Among our new cut-off values of FPG, 2h PG after 75 g OGTT, and HbA1C, 2h PG after 75 g OGTT showed highest sensitivity, specificity and accuracy (sensitivity, 85.7%; specificity, 98.6%; area under curve as meaning the accuracy of diagnosis, 0.945). FPG, using for screening test of diabetic diagnosis, showed lowest sensitive (sensitivity, 63.2%; specificity, 90.3%; area under curve as meaning the accuracy of diagnosis, 0.817). Conclusion: 2h PG after 75 g OGTT showed higher sensitivity for diabetes diagnosis in the elderly than FPG or HbA1C. Oral glucose tolerance test should be inspected to improve early diagnosis of diabetes mellitus in the elderly.