Abstract: Failure analysis of the fractured rear torsion beam of a certain vehicle in fatigue endurance test was done used direct reading spectrometer, hardness tester, scanning electron microscope and metallographic microscope. Chemical composition and Rockwell hardness of the rear torsion beam can meet the requirement of material standard and part drawing. Fracture and microstructure test results showed the fracture failure mode of the rear torsion beam was stress corrosion cracking. Poor anti-corrosion treatment of the inner wall caused corrosion which resulted in stress corrosion cracking and extending of the high strength and microstructure sensitive rear torsion beam under the tensile stress during the fatigue endurance test. Assembly with the rear torsion beam without corrosion was used to do the fatigue endurance test, and there was no cracking problem.