Objective: To determine the appropriate cutoff value for tumor maximum diameter of colorectal cancer and its relations with the clinicopathologic characteristics and prognosis of the patients. Methods: To clinical data of 119 patients with colorectal cancer undergoing radical resection followed by standard adjuvant chemotherapy from January 2006 to July 2012 were collected. The best cut-off value for tumor maximum diameter of colorectal cancer was determined by Kaplan-Meier survival analysis, the relations of tumor maximum diameter with clinicopathologic characteristics of the patients were analyzed, and the prognostic factors for colorectal cancer were also analyzed. Results: Using 4 cm as a cut-off point, the difference in survival rate of patients between two sides was most remarkable (65.5% vs. 51.1%, χ2=9.922, P=0.002), so 4 cm was regarded as the best cut-off value for tumor maximum diameter of colorectal cancer. There was significant difference in T classification, total number of lymph node detection, serum CEA level between patients with tumor maximum diameter <4 cm and ≥4 cm (all P<0.05). Univariate analysis showed that tumor maximum diameter, T classification, M stage, serum CEA level, and blood transfusion were related to the prognosis of the patients with colorectal cancer (all P<0.05), and multivariate analysis revealed that tumor maximum diameter, T classification and blood transfusion were independent prognostic factors for colorectal cancer (all P<0.05). Analysis after stratification by tumor size identified that T classification was an independent prognostic factor for colorectal patients with tumor maximum diameter ≥4 cm (HR=2.244, 95% CI=1.079–4.665, P=0.030), but all factors had no significant influence on prognosis of those with tumor maximum diameter <4 cm (all P>0.05). Conclusion: Tumor maximum diameter can be regarded as an independent prognostic factor for colorectal cancer, and the best cut-off value for tumor maximum diameter is 4 cm, which may be helpful for estimating the clinical features and prognosis of the patients.