NF-kB is one of the key transcription factors activated by receptor activator of NF-kB ligand (RANKL) during osteoclast differentiation. The 8-kDa dynein L chain (LC8) was previously identified as a novel NF-kB regulator. However, its physiological role as an NF-kB inhibitor remains elusive. In this study, we showed the inhibitory role of LC8 in RANKL-induced osteoclastogenesis and signaling pathways and its protective role in osteolytic animal models. LC8 suppressed RANKL-induced osteoclast differentiation, actin ring formation, and osteoclastic bone resorption. LC8 inhibited RANKL-induced phosphorylation and subsequent degradation of IkBa, the expression of c-Fos, and the consequent activation of NFATc1, which is a pivotal determinant of osteoclastogenesis. LC8 also inhibited RANKL-induced activation of JNK and ERK. LC8-transgenic mice exhibited a mild osteopetrotic phenotype. Moreover, LC8 inhibited inflammation-induced bone erosion and protected against ovariectomy-induced bone loss in mice. Thus, our results suggest that LC8 inhibits osteoclast differentiation by regulating NF-kB and MAPK pathways and provide the molecular basis of a new strategy for treating osteoporosis and other bone diseases.