The purpose of this study was to compare the resistance to vertical root fracture of two resin cements (Superbond and 3M Rely X ARC) and to observe the penetration of cement into the fracture surface. Thirty extracted human upper central incisors having approximately the same dentin thickness were used. The roots were sectioned from the crown to the length of 11.0 mm, and were endodontically prepared and filled with gutta-percha. The post spaces were prepared for cast metal posts. The roots with simulated periodontal ligament were embeded in the vilmix block. the specimens were mounted in an universal testing machine and loaded in compression until the root fractured. The fracture force was recorded in kiloNewton (kN). The fractured roots were randomly divided into 2 groups (n=15). The posts were reinserted and bonded with Superbond and 3M RelyX ARC, respectively. The bonded specimens were kept in 100% humidity for 24 hours, then were re-fractured and the force were recorded as re-fracture force. The mean fracture force was significantly higher than the mean re-fracture force (p<0.05). No significant difference between the re-fracture force after bonding with the two resin cements (p>0.05). Both resin cements can not completely penetrate into fracture line.The purpose of this study was to compare the resistance to vertical root fracture of two resin cements (Superbond and 3M Rely X ARC) and to observe the penetration of cement into the fracture surface. Thirty extracted human upper central incisors having approximately the same dentin thickness were used. The roots were sectioned from the crown to the length of 11.0 mm, and were endodontically prepared and filled with gutta-percha. The post spaces were prepared for cast metal posts. The roots with simulated periodontal ligament were embeded in the vilmix block. the specimens were mounted in an universal testing machine and loaded in compression until the root fractured. The fracture force was recorded in kiloNewton (kN). The fractured roots were randomly divided into 2 groups (n=15). The posts were reinserted and bonded with Superbond and 3M RelyX ARC, respectively. The bonded specimens were kept in 100% humidity for 24 hours, then were re-fractured and the force were recorded as re-fracture force. The mean fracture force was significantly higher than the mean re-fracture force (p<0.05). No significant difference between the re-fracture force after bonding with the two resin cements (p>0.05). Both resin cements can not completely penetrate into fracture line.