Abstract: A anti-roll torsion bar made of 51CrV4 spring steel for metro vehicles broken in service. The cause on fracture of torsion bar was analyzed by means of macroscopic observation, metallographic examination, SEM analysis, chemical composition analysis and mechanical property test. The results show that the fracture mode of torsion bar was normal mode torsional fatigue fracture. The strength of the base material of torsion bar was high and the toughness was low. Due to the local heating of torsion bar during the straightening, the contact part of the straightening tooling had local deformation, and the cooling speed was too fast, which formed the abnormal area of surface hardness on the middle section of torsion bar. Under the continuous action of torsional fatigue load, the fatigue crack source was formed with intergranular cracking as the main and quasi cleavage fracture as the auxiliary in the abnormal area of surface hardness, the crack expanded continually and caused the torsion bar fracture eventually.