Abstract: Objective The exosome has absorbed much attention in the study of cancers in recent years, and exosomal miRNAs have been validated to play important roles in variety of tumors. This study aimed to invest the serum exosomal miRNA-27a expression and its significance in gastric cancer. Methods By using real-time fluorescence quantitative PCR (QPCR), the serum exosomal miRNA-27a levels of both 40 gastric cancer and 20 normal people who were cured in our hospital from January 2015 to December 2017 were investigated; the serum exosomal miR-27a expression levels between the cancer group and control group were compared. Meanwhile, the change of serum exosomal miR-27a expression level of the 40 patients after the operation was compared with that before the surgery. The relevance of the expression level of serum exosomal miR-27a to the clinical pathological characteristics (sex, age and tumor grade) was invested, and the relationship between serum miR-27a and the prognosis of patients were analyzed based on the follow-up information. Results The serum exosomal miR-27a expression level of the 40 patients (0.153±0.092, before the surgery) was significantly lower than that of 20 normal control (0.823±0.125), the difference was statistically significant (P<0.001). After the operation, the level of serum exosomal miR-27a expression was 0.505±0.143, which was definitely up-regulated (P<0.05). Also, the expression level of serum exosomal miR-27a in the Ⅰ/Ⅱ grade patients was 0.179±0.102, and in the Ⅲ/Ⅳ grades patients was 0.102±0.076 (P<0.05), which revealed that the expression level of serum exosomal miR-27a was negatively correlated with the tumor grade in gastric cancer. Conclusion The expression level of miRNA-27a is definitely down regulated in gastric cancers and negatively correlated with its grade. The test of serum miR-27a level can provide an innovative target for the early scanning and treatments of gastric cancer.