Abstract: The chemical components, hardness, fractographs and microstructure of the fractured 45 steel suspension loop were analyzed by optical emission spectrometer, Brinell hardenss tester, scanning electron microscope and optical microscope. The results show that the fracture of the suspension loop was brittle fracture. Microstructure of the suspension loop was lamellate ferrite and coarsing grains, which show that the 45 steel was overheated and the impact toughness of it was decreased, and it was the main cause for brittle fracture of the suspension loop.