Objectives: To determine the effect of thrombin on human PDL cells by assay cellular proliferation and extracellular matrix synthesis. Since HPDL cells express PAR, a thrombin receptor, it is tempting to speculate that thrombin should involve in function and behavior of HPDL cells. Methods: Primary cultured of human periodontal ligament (HPDL) cells were incubated with various concentrations of thrombin for 48 hours ranging from 1 ng/ml to 100 ng/ml. Proliferation assay was performed by using colorimetric assay and the mRNA expressions of type I collagen and MMP-13 were determined by RT-PCR. Results: The result showed that at the concentration of 100 ng/ml, thrombin could significantly stimulate the growth of HPDL cells (P<0.05). In addition, at 10 ng/ml of thrombin, the upregulation of type I collagen and osteoprotegerin (OPG) was detected. The same concentration of thrombin could also downregulate the expression of MMP-13 and RANKL. Conclusion: The results indicate that thrombin can modulate the function of HPDL cells. The increase of proliferation and type I collagen with the reduction of MMP-13 suggest the role of thrombin in PDL tissue remodeling. In addition, the effect of thrombin on the balance of RANKL-OPG indicates the role of thrombin in the homeostasis of periodontal tissue.