Circulating miR-17-5p and miR-20a (miR-17-5p/20a) have been demonstrated to be elevated in the plasma of gastric cancer patients. However, the clinical significance of the circulating levels of these microRNAs (miRNAs), the predictive power for prognosis and application for monitoring of chemotherapeutic effects remain unclear. To this end, we measured plasma miR-17-5p/20a levels in unpaired pre-operative (n=65), post-operative (n=16) and relapse (n=6) gastric cancer patient groups. The 3-year overall survival rate for the unpaired pre-operative patients was recorded. The circulating levels of miR-17-5p/20a were also tested in paired pre-operative and post-operative plasma from 14 gastric cancer patients. We found that the concentrations of miR-17-5p/20a were significantly associated with the differentiation status and TNM stages of gastric cancer. The miRNA levels in the different groups reflected pathological tumor progression. Kaplan-Meier curve analysis revealed that high expression levels of miR-17-5p/20a were significantly correlated with poor overall survival. Cox regression analysis demonstrated that the level of plasma miR-20a was an independent risk predictor for prognosis. An in vivo mouse tumor model was established and antagomirs against miR-17-5p/20a were applied as chemotherapeutics to perform tumor treatment. An assay of serum miR-17-5p/20a levels showed that the levels of serum miRNAs were notably reduced in post-treated mice with tumor volume regression. Taken together, the levels of circulating miR-17-5p/20a may be a promising non-invasive molecular marker for pathological progression, prediction of prognosis and monitoring of chemotherapeutic effects for gastric cancer.