Objective：To measure the frequency of myeloid-derived suppressor cells（MDSCs） in peripheral blood and serum S100A9 level in patients with colorectal cancer（CRC），to analyze the correlation between the two indices，and to investigate their potential application in the clinical diagnosis and treatment. Methods：A total of 82 clinical peripheral blood samples （including 52 samples from patients with CRC and 30 samples from healthy subjects[as controls]） and related electronic medical records were collected. The frequency of MDSCs in peripheral blood mononuclear cells（PBMCs） was measured by flow cytometry. The serum S100A9 level was measured by a double antibody sandwich enzyme-linked immunosorbent assay. SPSS and Graphpad Prism were used for the Mann-Whitney test，Spearman R analysis，and receiver operating characteristic（ROC） curve analysis. Results：Compared with the controls，the patients with CRC had significantly increased frequency of MDSCs in PBMCs and serum S100A9 level（P<0.01），with a positive correlation between MDSC frequency and serum S100A9 level（r=0.38，P=0.01）. Compared with the patients who were at Dukes’ stage A/B and without any lymph node metastasis，the patients who were at Dukes’ stage C/D and with lymph node metastasis had significantly increased MDSC frequency and serum S100A9 level（P<0.01）；meanwhile，the area under the ROC curve was greater than 0.7 for both of the above indices（P<0.01），indicat－ing that both indices exhibited a favorable value in the diagnosis of CRC and the evaluation of the stage and metastasis of CRC. Conclusion：Patients with CRC have a significantly increased frequency of MDSCs in PBMCs and serum S100A9 level；therefore，both indices may be potential biomarkers that can contribute to the staging of CRC and predict the metastasis of CRC.