Objective: To investigate the alteration of the interleukin 6 (IL-6) level in the development of deep vein thrombosis (DVT) in rats and its actions. Methods: Sixty male SD rats were randomly divided into sham operation group (n=10) and model group (n=50). Rats in model group underwent bilateral femoral vein clamping plus plaster cast immobilization of the hind legs to induce hind limb DVT, and were sacrificed at 2, 5, 10, 15 and 25 h after operation respectively, with 10 rats in each time point, to obtain the vein samples. Then, the incidence of thrombus formation in the femoral vein specimens was observed, IL-6 mRNA level in the endothelial tissue of the femoral vein was measured by real-time PCR, and the serum levels of IL-6, plasminogen activator inhibitor (PAI) and tissue plasminogen activator (tPA) were determined by ELISA assay. Results: No thrombus formation was observed in rats in model group 2 h after operation, but thrombosis began to occur 5 h after operation and was increased with time. In rats of model group, the IL-6 mRNA level in the endothelial tissue of the femoral vein was gradually increased after operation, and reached the peak at postoperative 15 h; the alterations of both the serum IL-6 and PAI levels showed the same pattern of the alteration of the IL-6 mRNA level, while the serum tPA level presented the completely opposite changing pattern of the serum IL-6 and PAI levels. All the differences in above parameters at each observed time point in model group reached statistical significance in comparison with those in sham operation group (all P<0.05). Conclusion: IL-6 level is increased during the process of development of DVT in rats, and this may promote thrombosis through increasing PAI production and inhibiting tPA activation.