Abstract: The eyebolt on pneumatic diaphragm actuator of a control valve company applied in the petrochemical industry broke after fastened the eyebolts and nuts. The reason for the fracture was analyzed by chemical composition detection, mechanical property detection, microscopic morphology observation, microstructure observation and energy spectrum. The results show that the eyebolt presents brittle fracture with obvious cleavage fracture characteristics. The internal cracks, impurity calcifications, pits and other defects on the thread tooth top of the broken bolt were observed. The tensile strength is lower than the standard value. The value of loading force approaches or exceeds the tensile strength when the bolt is tightened. The cracking source forms at the part of the defects and the crack expands rapidly, which can result in fracture.