Abstract: Fracture of the gray cast iron counterweight occurs during bolt assembly. The fracture causes of the counterweight were analyzed by chemical composition analysis, metallographic analysis, energy spectrum analysis and mechanical properties test. The results show that there are casting defects such as shrinkage holes and porosities in the fracture of the counterweight, the brittleness of parts is increased by large amounts of phosphorous eutectic and carbides, stress concentration is caused by nonmetallic inclusions, which eventually cause brittle fracture of the counterweight during assembly.