ObjectiveTo quantitate expression of microRNA-21 (miRNA-21) in gastric cancer of different tumor stages and discuss its clinical value. Method The relative expressions of miRNA-21 were quantitated in the cancer tissues, corresponding normal gastric tissues adjacent to gastric cancer, and serums of 50 gastric cancer patients received opera-tion and confirmed gastric cancer by pathology and the serums of nongastric cancer patients in Xinhua Hospital Affiliated to Shanghai Jiaotong University School of Medicine and its Chongming Branch from January 2015 to January 2016 by real time quantitative PCR. ResultsThe relative expression level of miRNA-21 in the gastric cancer tissues was significantly higher than that in the normal gastric tissues adjacent to gastric cancer. Among the TNM stageⅠ, Ⅱ, Ⅲ of gastric cancer patients, the relative expression levels of miRNA-21 in the cancer tissues were 2.17 (1.48-2.90), 4.08 (2.30-4.86), 8.64 (5.82-18.20), respectively and the differences among these three stages were statistically significant (P<0.05). The relative expression level of the serum miRNA-21 in the gastric cancer patients was significantly higher than that in the nongastric cancer patients, which in the serums for stageⅠ, Ⅱ, and Ⅲpatients were 31.00 (24.60-37.15), 39.10 (28.90-39.80), 44.15 (38.95-56.68), respectively and the differences among three stages were statistically significant (P<0.05). The relative expression level of miRNA-21 in the serums and cancer tissues had a positive correlation (r=0.86, P<0.05). ConclusionMiRNA-21 appears to have a potential association with TNM stage of gastric cancer, which cautiously suggests that it might be a potential indicator for prediction of preoperative TNM stage of gastric cancer.