【Objective】Objective To evaluate the effect of heparin or heparin saline locking solution in maintenance hemodialysis (MHD) patients with tunnel-cuffed catheters. Methods After the right internal jugular vein catheters were removed in MHD patients due to various indications (except catheter dysfunction), the internal surface of different parts of the catheters were examined by electron microscopy. Results Visually, 41.7% of the catheters were completely occluded by thrombosis in the two lumens of catheters from side holes to tips of the catheters; 25% of the catheters had no obvious thrombosis; 33.3% were completely occluded by thrombosis in one lumen. Electron- microscopically, 100% of the catheters had thrombosis at the tips and side holes, especially obvious at the lateral holes and cutting edges of the tips. Spearman correlation analysis showed that thrombosis at the tips and lateral holes was related to hemoglobin level (r=0.488, P=0.047), but not related to gender (r=0.236, P=0.362), age (r=- 0.113, P=0.667), dilution of the locking solution (r=0.436, P=0.080), INR (r=- 0.339, P=0.183), platelet (r=- 0.187, P=0.473), total cholesterol (r=0.037, P=0.887), blood calcium (r=- 0.188, P=0.470), smoking (r=0.169, P=0.517), antiplatelet drugs (r=0.169, P=0.517), glucocorticosteroids (r=0.169, P=0.517) and catheter indwelling time (r=-0.305, P=0.234). The introducing hemoglobin level into the logistic equation had no statistically significance (OR=1.075, 95% CI 0.980～1.179, P=0.125), probably due to the insufficient number of cases. No obvious abnormalities were found in other parts of the catheters by scanning electron microscopy. Biofilm formation was found in 16.7% of the catheters. Conclusion The tip of the catheter is the site where thrombus is easily formed. The thrombosis at the tip and side hole of the catheters may relate to hemoglobin level.