Objectives: The American Diabetes Association (ADA) has recently recommended the HbA1c assay as one of four options for making the diagnosis of diabetes mellitus, with a cut-point of ≥6.5%. We compared the HbA1c assay and the fasting plasma glucose level for making the diagnosis of diabetes among Korean adults. Methods: We analyzed 8710 adults (age 45-74 years), who were not diagnosed as having diabetes mellitus, from the Namwon study population. A fasting plasma glucose level of ≥126 mg/dL and an A1c of ≥6.5% were used for the diagnosis of diabetes. The kappa index of agreement was calculated to measure the agreement between the diagnosis based on the fasting plasma glucose level and the HbA1c. Results: The kappa index of agreement between the fasting plasma glucose level and HbA1c was 0.50. Conclusions: The agreement between the fasting plasma glucose and HbA1c for the diagnosis of diabetes was moderate for Korean adults.