Abstract: Objective: To determine the effect of the PICC catheter/vein diameter (C/V) ratio on incidence of PICC-related venous thrombosis, and identify the optimal cut-off point of the C/V ratio. Methods: Adult patients were recruited between February 2016 and September 2016. Vein diameter at the PICC insertion site was measured using SiteRite 5 Ultrasound, and the vein was assessed by Doppler Ultrasonic examination. Participants were followed up at four weeks to determine if they developed PICC-related venous thrombosis. Receiver Operator Characteristic (ROC) was used to identify the optimal cut-off point of the C/V ratio. Results:Data were available for 247 patients. 98 patients (39.7%) developed PICC-related venous thrombosis. The differences of incidence of PICC-related venous thrombosis among different C/V ratios were significant (χ2=38.242, P<0.001). ROC curve determined that a 34% C/V ratio was the optimal cut off point (sensitivity=83.7%, specificity=71.8%). Conclusions:It was found that the C/V ratio was negatively related to the incidence of thrombosis. This study suggests that a 34% C/V ratio is the optimal cut off, and a 3Fr-PICC should be chosen if the vein diameter is (0.29~0.41) cm to prospectively prevent PICC-related venous thrombosis.