Background: Increased level of circulating myeloid-derived suppressor cells (MDSCs) have been associated with higher tumor stage, poorer survival, and poorer response to therapeutic agents in colorectal cancer (CRC). It has been reported that the common first-line chemotherapy involving combination with oxaliplatin mediated MDSCs depletion via induction of cell death. However, this phenomenon is yet to be further investigated since it only occurred in polymophonuclear MDSC (PMN-MDSC) subset, not in monocytic MDSC (M-MDSC). The present study aims to learn deeper on the dynamic changes of circulating MDSCs in response to oxaliplatin-based treatment in CRC patients. Methods: This was a prospective study that recruited 30 treatment-naïve patients with varying stage of CRC who were scheduled to receive oxaliplatin-based chemotherapy. Blood sampling was conducted prior and at several time points during and after chemotherapy. Multicolor flow cytometry assay was used to analyze the proportion of HLA-DR-, CD33+, and CD15+ (PMN-MDSC) and CD14+ (M-MDSC) cells within peripheral blood mononuclear cells (PBMCs). Other essential tumor biomarkers such as carcinoembryonic antigen (CEA) and tumor infiltrating lymphocytes (TILs) were assessed as well. As control, 14 healthy subjects were recruited. Results: Our result showed that circulating PMN-MDSCs was significantly higher in CRC patients compared to healthy subjects (p=0.003), whilst insignificant result was shown by M-MDSCs (p=0.890). Following chemotherapy, MDSCs level showed dynamic changes. Interestingly, a subgroup of CRC patients with decreased in both PMN- and M-MDSCs levels on D-14 chemotherapy were consistently showed a significant decreased of MDSCs level during and after therapy completion compared to baseline (p=0.0078). Conclusions: Circulating MDSCs level, particularly PMN-MDSCs in CRC patients was significantly higher compared to healthy subjects. Changes in both circulating PMN- and M-MDSCs levels at D-14 chemotherapy might have prognostic value in oxaliplatin-based chemotherapy.