Abstract: One T block of a chuck fractured during the commissioning period of a machine tool. To find out the fracture reasons, the T block of the chuck was tested and analyzed from the aspects of chemical composition, microstructure, hardness, fracture surface and force. The results show that the main fracture reasons of the T block of the chuck were its low strength of the material and the increased stress of the T block due to the unreasonable design of the claw. Eventually, the appropriate improvement measures were put forward according to the fracture reasons of the T block of the chuck.