Unilateral lumbosacral facet dislocation is very rare. In a review of literatures, only 14 cases have been documented. We could not find any report about this unusual injury in Korea. The mechanism of injury is hyperflexion and rotation. In general, attention is drawn to the association of fractured transverse process as an important clue to diagnosis. Attempts at closed reduction are futile, and the treatment of choice is open reduction and internal fixation with lumbosacral fusion.