Abstract： Objective:To analyze the differential expression of breast milk-derived extracellular vesicles (BM-EV) from mothers of preterm and term infants .Methods:Breast milk samples were collected from preterm and term delivery (three cases in each) at the Women's Hospital of Nanjing Medical University in 2019. BM-EV was extracted using ultracentrifugation. After preliminary identification of the characteristics of BM-EV, liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used for protein quantification. Significantly up-regulated differential proteins (fold change≥1.5 and P<0.05) in the preterm group were screened. GO and KEGG were performed to predict the differentially expressed proteins' functional annotation and determine associated signaling pathways. Mann-Whitney U test and Fisher's exact test were used for intergroup comparisons. Pearson's correlation test describes the correlation of protein quantification values between samples. The differences in protein abundance were compared between the two groups using a t-test, followed by multiple corrections. Additionally, significantly enriched GO terms and KEGG pathways of the differentially expressed proteins were screened based on the hypergeometric distribution. Results:(1) There were three primiparae in the preterm group and one in the term group. Marker proteins CD9, CD81, and HSP70 were enriched in the BM-EV of both groups. (2) Six samples were comparable between groups and showed high reproducibility within groups. The correlation of protein quantification values between samples was up to 0.99. Furthermore, the coefficient of variation was 11.21% for preterm samples and 19.72% for term, and the data values in the preterm group were relative. (3) A total of 945 proteins were identified, and 156 were differentially expressed between preterm and term BM-EV, with 83 significantly up-regulated in preterm BM-EV. In the up-regulated proteins, the top three high-abundance proteins were complemented C4a, fatty acid synthase, and sclerostin domain-containing protein-1. (4) The biological processes or cellular components with the highest enrichment in GO functional prediction were mainly involved in hemoglobin and glycogen biosynthesis, immunological synapse formation, and phagocytosis mediated by the Fc γ receptor signaling pathway. The most relevant KEGG pathways were ribosome-related, complement and coagulation cascades, neutrophil extracellular trap formation, and Fc γ receptor-mediated phagocytosis.Conclusion:The significantly up-regulated differential proteins in BM-EV may play a protective role by regulating immunity, gastrointestinal function, and energy metabolism in preterm infants.