Objective: The aim of this study was to evaluate the usefulness of 100-g oral glucose tolerance test (OGTT) at mid-pregnancy on postpartum glucose intolerance at 6~12 weeks after delivery in women with gestational diabetes mellitus (GDM). Methods: We enrolled 1,198 pregnant women diagnosed with GDM from October 2005 to December 2016. GDM was diagnosed by two step approach with a 50-g screen followed by a three-hour 100-g OGTT for those who screen positive. 75-g OGTT was performed at 6~12 weeks after delivery and postpartum glucose intolerance was defined as FPG ≥ 100 mg/dL or 2-h PG ≥ 140 mg/dL. Results: Mean age was 33.2 ± 3.6 years and mean pre-pregnancy body mass index was 22.4 ± 3.6 kg/m2. The prevalence of postpartum glucose intolerance increased with increasing numbers of abnormal value during 100-g OGTT (2 = 44.0%, 3 = 51.3%, and 4 = 65.6%, P for trend < 0.05). Women with 3 and 4 abnormal values of 4 tests were 1.30 times (95% CI 1.00-1.69) and 2.24 times (95% CI 1.62-3.10) more likely to have postpartum glucose intolerance than those with 2 abnormal values of 4 tests. Among the components of 100-g OGTT, hyperglycemia at fasting and 3-h test were risk factors for postpartum glucose intolerance, even after adjusting for maternal age and pre-pregnancy body mass index (hyperglycemia at fasting test OR 1.03, 95% CI 1.03-1.65; hyperglycemia at 3-h test OR 1.52, 95% CI 1.19-1.93). Conclusion: In women with GDM, more than 3 abnormal values of 4 tests during 100-g OGTT at mid-pregnancy are associated with elevated risk of postpartum glucose intolerance. Consequently, 100-g OGTT should be performed completely and we pay attention to manage postpartum glucose intolerance when women with GDM have more than 3 abnormal values of 4 tests during 100-g OGTT at mid-pregnancy.