Objective To prospectively study the abundance and diversity of bacterial flora in the colostrum of postpartum women with gestational diabetes mellitus（GDM） and normal postpartum women. Methods According to the inclusion criteria，we enrolled 30 postpartum women with GDM and 30 healthy postpartum controls，all undergoing a cesarean delivery. Colostrum samples were collected from each subject to isolate DNA for high-throughput sequencing of the 16s rRNA V3-V4 amplicon using the Illumina MiSeq platform. Results The abundance-based coverage estimator for evaluating microbial richness was significantly different between the normal group and the GDM group（P=0.039）. The two groups showed no significant difference in alpha diversity，but differed significantly in beta diversity（P=0.001）. Compared with the control group，the GDM group showed lower mean relative abundance levels of the Firmicutes and Proteobacteria phyla，Streptococcaceae and Gemellaceae families，and Streptococcus and Roseburia genera； and higher mean relative abundance levels of the Actinobacteria phylum，Staphylococcaceae family，and Staphylococcus genus in the milk. Conclusion The abundance and diversity of colostrum microbiota are different between GDM mothers and healthy mothers，and the decreased relative abundance of Firmicutes and Proteobacteria in the breast milk of women with GDM may influence the establishment of gut microbiota and the growth and development of their children in the early stage. Quantifying the composition of breast milk microbiota has special significance for GDM mothers and their offspring.