Aims: The coagulation disorders leading to the loss of both whole organ xenografts have increasingly been recognized as a critical barrier to successful transplantation. Coagulation is initiated by damage to blood vessel endothelium and the expression of tissue factor (TF). The aim of this study is to determine the effect of TF gene knockdown on pig aortic endothelial cells (SVAP) in coagulation disorders in vitro. Methods: SVAP were transfected with siRNA specific for TF or a nonspecific control siRNA using lipofectamine 2000. Transfected SVAP were then analyzed for TF gene and protein expression by real-time PCR and FACS respectively. The effect of TF knockdown in SVAP on anti-coagulation was evaluated by incubation with human blood for 60 min. Clots were collected and measured. Platelets, leukocytes and neutrophils were counted by using Beckman Coulter ACT. Results: TF knockdown resulted in substantially reduced production of TF by SVAP. TF gene level of siRNA transfected SVAP was 22± 4% (n = 4) of naïve SVAP which was determined by real-time PCR. FACS results proved TF protein level of siRNA transfected SVAP had been reduced by 70%(57.1± 5.4% vs13.3±2.4 %, n = 4, p < 0.01). Nonspecific siRNA had no effect (53.6±4.5%, n = 4). When TF siRNA transfected SVAP were incubated with human blood there was no obvious reduced clot weight (0.42±0.04g vs 0.32±0.05g, n=5, p>0.05) and reduced consumption of platelets (4.1±1.2×10^9/L vs 6.5±1.3×10^9/L n=5, p>0.05), leukocytes (2.3±0.3×10^9/L vs 2.8±0.5×10^9/L, n=5, p>0.05) and neutrophils (0.94±0.06×10^9/L vs 1.22±0.06×10^9/L, n=5, p>0.05) when compared to that detecte in the well with non-transfected SVAP. Conclusion: The expression of TF on endothelial cells is involved incoagulation reaction but not the major coagulation factor in xenotransplantation. Key words: Tissue factor, pig aortic endothelial cells, Anticoagulation, Xenotransplantation The study was approved by the Sydney West Area Health Service Human and Animal Research Ethics Committees.