Objective: The current diagnostic criteria for diabetes were established based on the fasting glucose, 2-hour postprandial glucose and HbA1C. However, 30-minute postprandial glucose (PP30), which reflects an early phase insulin secretion, had not been used for clinical implications. The aim of this study was to access the difference in index of β-cell function, metabolic factors and progression to type 2 diabetes (T2DM) according to PP30 level. Methods: A total 1,888 subjects, who had suspected abnormal glucose tolerance or family history, were performed 75-g oral glucose tolerance tests (OGTT) at time 0, 30 and 120 min. All subjects were classified as having normal glucose tolerance (NGT, n = 141), prediabetes (IGT or IFG, n = 688), or T2DM (n = 1059) according to the OGTT results. We calculated insulinogenic index and HOMA-IR. We identified the characteristics of prediabetes with PP30 over 200 mg/dl compared to those with PP30 below 200 mg/dL. Results: Newly diagnosed T2DM and prediabetetic subjects had significantly lower insulinogenic index (0.19 ± 0.2 vs 0.49 ± 0.8 vs 0.87 ± 0.6, P < 0.001) and higher HOMA-IR (4.0 ± 4.4 vs 3.0 ± 1.8 vs 1.9 ± 1.5, P < 0.001) and aggravated metabolic factors (BMI, lipid profile) compared to subjects with NGT. In addition, prediabetes with PP30 over 200 mg/dL had lower insulinogenic index (0.32 ± 0.6 vs 0.58 ± 0.66, P < 0.001) and higher HOMA-IR (3.3 ± 2.5 vs 2.9 ± 1.5, P = 0.023) compared to those with lower PP30. However, there were no significant differences in metabolic factors according to PP30 level. 16% of prediabetic subjects progressed to T2DM and mean intervals to progression were 30.9 months and mean follow-up duration were 62.1 months. While 14.3% of prediabetes with PP30 below 200mg/dl progressed to T2DM, 21.4% of prediabetes with PP30 over 200 mg/dL progressed to T2DM. Conclusion: Prediabetes with PP30 over 200mg/dl showed β-cell dysfunction and insulin resistance similar to T2DM. This study suggests that prediabetic subjects with high PP30 should be monitored closely.