Objective To study the modes of fragmentation of superior central incisor restored by cast post-and-core system.Methods 32 human maxillary incisors extracted within three months were randomly divided into four groups,then they were endodontically treated.The author amputated the crowns of these teeth,then restored all these teeth with Ni-Co cast metal post-core-crown system.The diameters were 1.10,1.50,1.70 and 2.00 mm.After that the roots were embedded with plaster and loaded on Universal testing machine at a 135-degree angle to the long axis of the roots until the roots fractured.Results were analyzed statistically with SAS statistical software.Results There was statistically significant difference of fracture loads among four groups.We found the trend that the modes of fragmentation were from the distortion of posts to the roots’ diagonal fracture of upper 1/3 to vertical fracture of roots with the diameters getting more larger.Conclusion During restoration of maxillary central incisor by cast post-and-core system,the thinner post is benefit to protect the root tissue.