Background: Gestational Diabetes Mellitus (GDM) is a complex metabolic condition that has a detrimental effect on both maternal and foetal outcomes. Oxidative stress contributes to the pathogenesis of GDM by causing insulin resistance in peripheral tissues and affecting insulin production from pancreatic β cells. Oxidative stress is defined as an increase in free radical activity and lipid oxidation levels. GGT could be utilised as an indicator of the GDM. Objective: To measure serum gamma-glutamyl transferase (GGT) in first, second and third trimester of pregnancy. Materials and Methods: This study was a cross sectional study was conducted the department of Clinical Pathology in collaboration with the department of Obstetrics and Gynecology, BSMMU, Dhaka from July 2015 to June 2016. Total 149 pregnant women at 1st, 2nd and 3rd trimester of pregnancy. The WHO criteria for diagnosing gestational diabetes mellitus (GDM) in 2015 include fasting plasma glucose 5.1-6.9 mmol/L and a 2-hour plasma glucose level of 8.5-11.0 mmol/L after a 75g oral glucose tolerance test (OGTT). These criteria are used to identify women at risk of adverse neonatal outcomes due to gestational diabetes were included in this study. Women with previous diagnosis of DM, systemic disease (HTN, Collagen tissue disease, Heart disease. Renal disease, Chronic liver disease) and history taking Alcohol and some drugs that affects GGT (Phenytoin, Phenobarbital, Acetaminophen, HMG CO reeducates inhibitor, OCP) were excluded in this study. Results: The mean OGTT was found 4.43±0.50 mmol/L in first trimester and 5.17±12.9 mmol/L in second trimester and 6.60±1.26 mmol/L in third trimester respectively. That were statistically significant between each group. Mean blood glucose after 2 hours was 5.59±1.11 mmol/L and 7.25±1.87 mmol/L in first and second trimester and 6.60±1.26 mmol/L in third trimester respectively. That were statistically significant between each group. Mean GGT was 23.36±6.89 IU/L in 1st trim