Although women with gestational diabetes mellitus (GDM) have an increased risk of having large-for-gestational age (LGA) infants, the association between LGA and antenatal hemoglobin A1c (HbA1c) levels is inconsistent among studies. The difference in study population and timing of HbA1c measurement may result in this variability. HbA1c is accepted as a standard tool for determining average blood glucose levels over the past 2 months and hyperglycemia in women with GDM usually start around the end of second trimester of pregnancy, HbA1c measurement at around gestational age of 36 weeks is therefore reasonable for evaluating glycemic control in these patients. In addition, HbA1c levels also vary among different ethnic groups. This study aimed to evaluate the association between HbA1c levels at around gestational age of 36 weeks and the risk of having LGA infants in Thai women with GDM. Singleton pregnant women diagnosed as GDM by the Carpenter and Coustan criteria and having HbA1c measurement at 36±2 weeks of gestation were included in this retrospective study. Those with known diabetes mellitus prior to pregnancy were excluded. LGA was defined as a birthweight over 90th percentile for gestational age and macrosomia as a birthweight over 4000 g. Logistic regression was used to evaluate the association of HbA1c levels and LGA or macrosomia. A total of 971 women with 1042 pregnancies were eligible for study. The median (IQR) of HbA1c levels was 5.2% (4.9, 5.5). The prevalence of LGA and macrosomic infants was 20.2% and 2.9%, respectively. The adjusted ORs (95%CI) of LGA and macrosomia for each 1% increase in the HbA1c level were 2.35 (1.70, 3.25) and 4.61 (2.37, 8.94), respectively. In conclusion, there was a significant association between HbA1c levels at around 36 weeks of gestation and the risk of having LGA infants in Thai women with GDM.