Background: Gestational Diabetes Mellitus (GDM) is the most common metabolic complications of pregnancy, and causes fetal mortality and morbidity. Therefore, early diagnosis of GDM is necessary to reduce maternal and fetal morbidity and to help prevent or delay the onset of type 2 diabetesObjective: This prospective study was carried out to determine the incidence of GDM in Yazd and to assess the effect of various contributing factors.Materials and Methods: One thousand and seventy one pregnant women were screened for GDM at 24-28 weeks. Initial screening was done by a glucose challenge test with 50 g glucose. If the 1-hour blood glucose level exceeded 130 mg/dl, then a 3-hour oral glucose tolerance test (OGTT) with 100g glucose was performed and diagnosis was established according to American Diabetes Association criteria.Results: Three hundred and forty two (31.9%) women had an abnormal screening test and proceeded to oral glucose tolerance testing. The overall incidence of GDM was 10.2% (n=110). Seventy six of subjects (7.1%) have one abnormal OGTT. There was a significant association between incidence of GDM and age, familial history of diabetes, BMI before pregnancy, parity, history of GDM, macrosomic baby, still birth during previous pregnancies and systolic and diastolic blood pressure. Conclusion: According to high incidence of GDM in our area we recommend screening for GDM in all pregnant women and modification of contributing factors in high risk women.