The estrogen receptor α (ERα) gene is a potential candidate gene of gestational diabetes mellitus (GDM).The purpose of the study was to investigate the relationship of ERα gene polymorphism (single nucleotide polymorphism [SNP]) and its expression in placental tissues with the development of GDM.The SNPs of PvuII and Xba I in the ERα gene of 175 pregnant women with GDM and 240 healthy pregnant women were detected by polymerase chain reaction-restriction fragment length polymorphism. Immunohistochemistry and western blotting were used to analyze the expression of the ERα gene in placental tissues.The results showed that the frequency of the CC + CT genotype and the C allele frequency of PvuII in the GDM group was significantly higher than that of the control group (p < 0.05). There was no significant difference in the genotype distribution and allele frequency of Xba I between the GDM group and control group. The expression of ERα in placental tissues of pregnant women with GDM was higher than that in the control group (p < 0.05). The participants with the PvuII CC + CT genotype had elevated levels of fasting blood glucose, homeostasis model assessment of insulin resistance (IR), and ERα expression in placental tissues compared with those with the TT genotype in the GDM group (p < 0.05). The SNP of Xba I of ERα gene had no correlation with clinical biochemical indicators of GDM and the expression of ERα in placental tissues (p > 0.05).This study suggested that SNP of the ERα gene and abnormal expression of ERα in placenta tissues were associated with GDM. The C allele of PvuII may be associated with GDM. In addition, SNP of the PvuII site in pregnant women with GDM was related to the degree of IR and to the upregulation of ERα expression in placental tissues, which may play an important role in the pathogenesis of GDM.