Abstract:  A completely preserved mandible of Leptobos from the Early Pleistocene mammalian fauna of Longdan, Gansu Province, China, is described here. The mandible has a long thin mandibular body, and a long mandibular diastema. The premolar row is short. The mandibular body forms an obtuse angle of about 120° with the mandibular ramus, while the angle of the mandible is nearly 90°. The paraconid and parastylid are well developed, and the metaconid extends posteriorly, not connecting with the paraconid in p3 or p4. The main cusps of the molars are rounded, and ectostylids (basal pillars) present in m1-m3. The metastylid of m2 is weak, and the preprotocristids of m2-m3 and posthypocristid of m3 are anteroposteriorly constricted. Skulls of Leptobos brevicornis were previously discovered in the same area, and the new specimen is also attributable to this species. Wounds preserved in the anterior part of the mandible are interpreted as resulted from attack by a feline predator, indicating the predatory behavior similar to that of living big cats, using muzzle clamps to suffocate preys, may have already occurred in the Early Pleistocene.