Abstract: Background and Objective: Gestational diabetes mellitus (GDM) is a common metabolic complication of pregnancy and associated with increase rates of maternal and perinatal problems. Irisin is a novel adipomyokin which plays a role in carbohydrate and fat metabolism, insulin response and inflammatory response. To assess serum level of irisin, cut off level and its correlation with different anthropometric and biochemical parameter in pregnant women with GDM. Materials and Methods: case control study conducted at AL-Yarmouk teaching hospital/Al-Mustansiriyah medical college in the period from April, 2017-April, 2018. One hundred pregnant women with comparable age, gestational age and body mass index were enrolled, 50 pregnant women with GDM (study group) and 50 normal pregnant women (control group). Serum irisin level was measured by enzyme-linked immunosorbent assay kit at 24-28 weeks of pregnancy and assessed in correlation with BMI, Hb A1c, fasting insulin and Oral Glucose Tolerance Test (OGTT). Results: Serum Irisin level was significantly lower in gestational diabetic women in comparison with healthy pregnant women (1.6±0.4 vs. 2.6±0.7 μg L–1) with p-value <0.001.It was independently low in correlation with BMI, serum insulin and OGTT. The cutoff value of serum Irisin in determining gestational diabetes was 2.145 μg L–1 with overall 87.0% accuracy. Conclusion: Serum irisin was significantly and independently low in women with GDM. Its cutoff level may be used as biochemical marker in the diagnosis and screening of GDM.