Objective Prediabetes includes impaired fasting glucose (IFG) or impaired glucose tolerance (IGT). To evaluate more reliable predictive values for diabetic conversion, we compared the incidence of diabetes in each category of prediabetes. We also analyzed which glycemic parameters including 2 horal glucose tolerance test (OGTT) and which insulin related indices were associated with conversion of diabetes. Methods Prediabetes (n = 577, 121 IFG, 237 IGT and 219 IFG & IGT) were assessed with glycemic parameters and insulin indices. We performed 2-h 75 g OGTT among 208 patients. They were followed for up to 9 years for conversion of diabetes. Several glycemic parameters, insulin, c-peptide, indices related to beta cell dysfunction and insulin resistance were compared in diabetic converter and non-diabetic converter. Results Ninety-three patients were diagnosed diabetes (mean duration 32.7 ± 23.1 months). In IGT and IGT & IFT, the rate of diabetes conversion was higher than that of IFG. Mean glucose during a 2h-OGTT was higher (176.6 ± 21.2 vs. 151.9 ± 21.0, P < 0.001) and 30-min insulin was lower (28.3 ± 13.8 vs. 49.6 ± 29.6, P < 0.001) in diabetes conversion group. There was no diabetic converter among prediabetes with 2h-OGTT with mean glucose lower than 150 mg/dL or 30-min insulin higher than 50 μU/mL. Indices related to beta cell dysfunction were significantly related to diabetes convertion, while insulin sensitivity indices were not. Conclusion IGT than IFG is better prediabetes category for predicting diabetes conversion. This might be due to decreased beta cell function in Korean prediabetes. In this study, beta cell function related indices, but not insulin sensitivity reltated indices showed singificant differences between diabetic converters and nonconverters. Higher mean glucose and lower 30-min insulin was also observed in diabetes converters. (The National Project for Personalized Genomic Medicine (PGM21), Grant number A111218-CP02)