Mechanical stress plays an important role in bone remodeling. However, it is still unclear whether mechanical stress regulates osteoclastogenesis mediated by mesenchymal stem cells (MSCs) during initial osteodifferentiation. We investigated the effects of static and dynamic pressures on osteoclast-inducing potential of MSCs during early osteodifferentiation. The osteoclastogenesis was examined using TRAP staining. The mRNA levels of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) genes were analyzed using real-time RT-PCR. It was shown that MSCs exposed to either pressure during initial osteodifferentiation promoted osteoclastogenesis with the up-regulation of RANKL/OPG ratio. MSCs displayed diverse responses to pressures at different points of initial osteodifferentiation. The RANKL/OPG ratio was significantly increased after osteoinduction in the primary MSCs without pressures exposure, which contradicted the previous report. These results suggest novel mechanisms of the initial biological responses of bone remodeling upon mechanical stimuli.