Objective: The diagnosis of gestational diabetes mellitus (GDM) is an important issue in terms of prevention of maternal and fetal complications. In our study we aimed to evaluate the relation of HbA1c and blood glucose levels of 75 and 50-100 gram oral glucose tolerance test (OGTT) in pregnant patients who were screened for GDM. Materials and methods: The parameters of 913 pregnant women screened for GDM are evaluated retrospectively. The two steps screening with 50-100 gram OGTT were used in 576 patients. The remaining 337 patients were screened with 75 gram OGTT. Results: The HbA1c levels of patients having high blood glucose (>= 153 mg/dl) levels at 2nd hour in 75 gram OGTT were significantly higher than patients having normal blood glucose levels at 2nd hour of 75 gram OGTT (P=0.038). Correlation analyses showed no significant relation between any blood glucose level of 100 gram OGTT and HbA1c level. Whereas in 75 gram OGTT 1st and 2nd hour blood glucose levels were found to have a significant relation with A1c levels (P=0.001, P=0.001 respectively). Conclusion: HbA1c may be used as an important tool in the diagnosis of GDM. But due to the variation of HbA1c in pregnant women and there is not an absolute cut-off level for A1c, it may be more reliable to evaluate HbA1c level together with the blood glucose levels in OGTT.