Ceratopsians underwent great changes including the shift of the locomotion mode, enlarged horns and frills, and increased body size. These changes are along with the changes in endocranial morphology and ecology such as the decrease of the flocculus, the hearing range, the olfactory ratio, and the Reptile Encephalization Quotient. However, the endocranial structures and associated ecology of the earliest ceratopsians are still unknown. Here, we reconstructed the endocasts of three early-diverging ceratopsians including the Late Jurassic Yinlong, and the Early Cretaceous Liaoceratops and Psittacosaurus. These ceratopsians display prominent flocculus, large and separate olfactory bulbs, long and high anterior semicircular canal, and long endosseous cochlear duct. Unlike Liaoceratops and Psittacosaurus, Yinlong exhibits relatively stocky semicircular canals and small flocculus. In Ceratopsia, the evolutional patterns of the endocast include the merging and narrowing of the olfactory bulbs, the disappearing flocculus, the reduction of the anterior semicircular canal, the increasing of the angle between two vertical semicircular canals, and the shortening of the cochlear duct. The endocranial structures suggest early-diverging ceratopsians had a high olfactory acuity and were adapted to hearing high frequencies, unlike the condition of late-diverging ceratopsians with lower olfactory acuity and hearing frequency. Reptile Encephalization Quotient suggests that Yinlong and Psittacosaurus were more encephalized than most extant reptiles and late-diverging ceratopsians. The angle of the lateral semicircular canal suggests that heads in ceratopsians display a transition from an upward posture to a downward posture. The relatively upward head posture indicates that early-diverging ceratopsians could feed more selectively such as to eat some leaves.