BACKGROUND: By evaluating serum Ribonucleotide-diphosphate Reductase subunit M2 (RRM2) levels and mRNA tissue expression, we aimed to investigate the potential role of RRM2 as a prognostic biomarker in Colorectal Cancer (CRC) patients. METHODS: This descriptive-analytic cohort study was conducted on 50 newly diagnosed CRC patients (stage II, III). Real-time PCR determined the mRNA tissue expression of RRM2. Fifty healthy individuals who came to the hospital of Golestan University of Medical Sciences and Tehran University of Medical Sciences for routine check-ups were considered the control group. Serum RRM2 protein was measured using an ELISA method in the patient group before, one month, and three months after the surgery, and in the control group just on the day of a routine check-up. The tumor metastasis node (TMN) classification system and occurrence of liver metastasis were evaluated in CRC patients. RESULTS: The RRM2 gene expression ratio and 95% confidence interval (CI) of the cancerous tissue was 6.56 times higher than the normal tissue (p < 0.001). Blood Sugar level (BS) (p < 0.001) and platelet level (PLT [range 0.004-499 × 10 /mm]; p = 0.010) were higher in the case group compared with the control group significantly, while Mean Corpuscular Volume (MCV) was significantly lower in the case group (p = 0.015). Overall, the mean serum of RRM2 protein levels in patients was remarkably diminished from before surgery until three months after surgery (p < 0.001). CONCLUSION: Serum RRM2 level and mRNA expression were significantly higher among CRC patients which could be considered a biomarker regarding CRC progression.