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Background: Gestational diabetes mellitus (GDM) is common problem during pregnancy. Diagnostic criteria of this problem are based on foreign population. Because of differences in racial, cultural, and nutritional characteristics, we need to determine these criteria are suitable for Iranian population. Objective: To determine whether different diagnostic criteria of gestational diabetes mellitus (GDM) are suitable for Iranian population. Materials and Methods: Prospective study was performed on 617 pregnant women. 1804 subjects referred for 50 g glucose challenge test (GCT) between 24th and 28 th weeks of gestation. 617 women with abnormal GCT (blood glucose ≥130 mg/dl) underwent 100-g 3-h oral glucose tolerance test (OGTT). The results were classified by three diagnostic criteria: new “Iranian ” diagnostic criteria based on the results from the 100-g 3-h OGTT performed in healthy participating women; the Carpenter and Coustan (CC) criteria; and the National Diabetes Data Group (NDDG) criteria. Obstetric and neonatal outcomes were recorded