Objectives: Determining serum cobalamin levels in Pregnant Women suffering from Gestational Diabetes mellitus (GDM) presenting at our tertiary care hospital. Study Design: Case control study. Setting: Department of Gynecology and Medicine, SMBB Medical College Layari General Hospital Karachi. Period: January 2016 to April 2017. Material & Methods: Sample of 100 pregnant women in 2nd and 3rd trimester was selected into; 50 controls and 50 GDM cases through convenient sampling. GDM was defined as pregnant women with fasting blood sugar ≥100 mg/dL. 5 ml blood was collected; 3 ml put into EDTA tubes for complete blood counts and 2 ml for sera. Blood glucose was estimated by hexokinase method, HbA1c by colorimetric method and cobalamin by ECLIA assay method. SPSS software 21.0 (IBM, Inc USA) was used for data analysis using Student t-test and Chi-square test (P ≤ 0.05). Results: Age of control was 30.43±1.49 years and 29.95±1.27 years in cases. Gestational age was 33.67±2.69 weeks in controls and 34.75±2.53 weeks in cases. Control and cases shows serum cobalamin levels of 316.34± 113.77 pg/ml and 253.5±121.32 pg/ml respectively (P=0.009). Serum cobalamin deficiency was noted in 68% of cases and 40% of controls (P<0.05). Glycemic control was bad in majority of cases. Serum cobalamin shows inverse correlation with random blood glucose, fasting blood glucose and Glycated HbA1. Conclusion: We found low serum cobalamin levels in pregnant women suffering from gestational diabetes mellitus that showed inverse correlation with random and fasting blood glucose and glycemic control.