Objective: This study aimed to evaluate the effects of tumor necrosis factor (TNF)-α on receptor activator of nuclear factor-κB (RANK) expression in osteoclast precursors in vitro and during orthodontic tooth movement (OTM) in vivo. Design: We assessed whether TNF-α influenced RANK expression levels in osteoclast precursors in vitro by real-time PCR and western blotting. For in vivo experiments, TNF-α was subcutaneously injected into mouse calvariae daily for 5 days. Mice were sacrificed and RANK expression was evaluated by real-time PCR and immunohistochemistry. For OTM, a nickel-titanium closed-coil spring was fixed between the upper incisors and upper-left first molar to move the first molar in the mesial direction in wild-type (WT) and TNFR1/TNFR2-deficient (TNFRsKO) mice. After OTM, the number of RANK-positive cells on the compression side was evaluated by immunohistochemistry. Results: RANK levels were enhanced in TNF-α-treated osteoclast precursors in vitro. RANK mRNA expression levels and the number of RANK-positive cells were higher in TNF-α-injected mice than in phosphate-buffered saline-injected mice. RANK-positive cells increased on the compression side of the alveolar bone in WT mice because of the mechanical loading. In addition, the number of RANK-positive cells on the compression side was significantly higher in WT mice than in TNFRsKO mice after OTM. Conclusion: These results suggest that TNF-α induces RANK expression in vitro and at baseline in vivo, as well as on the compression side during OTM.