This study was designed to determine serum human leukocyte antigen-G (HLA-G) levels and establish whether serum HLA-G level is related with gestational diabetes mellitus (GDM). Twenty-five GDM patients aged between 24 and 34 years and 24 healthy pregnant women aged between 22 and 33 years were included in this study. Health status of subjects was determined by medical history, physical, and obstetric examinations. Absence of family history for Type 2 diabetes mellitus, absence of diagnosis or clinical evidence of any major disease, and absence of medication use altering glucose metabolism constituted the inclusion criteria. GDM cases who are at their 24-28 weeks of pregnancy and who have not been using any oral hypoglycemic agents were included in GDM group. Control group consisted of healthy pregnant women at their 24-28 weeks of pregnancy. Women with GDM had significantly lower levels of serum HLA-G than controls. HLA-G levels were negatively correlated with all parameters of glucose metabolism including insulin (r=- 0.14; p=0.338), fasting blood glucose (r=- 0.220; p=0.129), 1-h oral glucose tolerance test (OGTT) blood glucose (r=- 0.18; p=0.271), 2-h OGTT blood glucose (r=- 0.314; p=0.172), homeostasis model assessment (r=- 0.226; p=0.119), HbA1C (r=-0.342; p=0.108), and WBC (r=-0.149; p=0.307). There was a negative correlation between HLA-G and BMI (r=-0.341; p=0.016). Decreased circulating HLA-G level is found to be associated with insulin resistance and GDM, in this study.