〔Abstract〕 Objective To explore the influential factors on tumor diameters in rectal cancer. Methods Cross sectional study data were collected from patients undergoing radical resection of rectal cancer.Patients were grouped by median value of tumor diameter, and the clinical pathological characteristics were compared between the two groups.The associations between tumor diameter and continuous variables were analyzed using locally weighted scatterplot smoothing(LOWESS) method.The associating factors of tumor diameter were further analyzed using generalized additive model(GAM). Results Patients with rectal cancer were divided into two groups with tumor diameter either<4 centimeters or ≥4 centimeters.Proportions of T,TNM stages differed significantly between different groups.Median carcinoembryonic antigen(CEA) in≥4 centimeters group were 4.90 ng/ml(inter-quartile range [IQR] 2.39,9.53 ng/ml),much higher than that in<4 centimeters group(2.93 ng/ml [IQR:1.78,4.90 ng/ml]),P<0.01.LOWESS analysis showed that there were non-linear relationships between tumor diameter and the following variables: age, the distance between tumor and anal verge, CEA,neutrophils, lymphocytes, platelets, neutrophil-to-lymphocyte ratio, and platelet-to-lymphocyte ratio.GAM showed that increased blood platelets(P<0.05) and later T stage(P<0.01) were significantly associated with increased tumor diameters, and increased CEA levels were also associated with larger tumors(P<0.01),but there were saturation effects. Conclusion Platelets, T stage and CEA are associated with tumor diameter in rectal cancer, indicating that the above factors should be considered when studying associated factors of tumor diameter in rectal cancer.