Objective: To demonstrate the simultaneous expression of cathepsin K mRNA and cathepsin K protein in rat root-resorbing tissue induced by tooth movement, to identify the cells that produce cathepsin K, and to postulate the role of cathepsin K in resorptive activity. Methods: Tooth movement was performed to induce the root resorption in rat maxilla by stretch of Ni-Ti spring coil for 7 days. The localization of cathepsin K mRNA and protein were detected by in situ hybridization and immunohistochemistry. Results: Cathepsin K mRNA and protein were highly expressed in odontoclasts and osteoclasts in root-resorbing tissue, resorptive multinucleated osteoclasts and odontoclasts were aligned along the surface of the bone, bone resorption lacunae were formed. Conclusion: Cathepsin K is secreted by osteoclasts and odontoclasts, it plays a vital role in root resorption activity.