Abstract: The cause for failure of the 45°hot-bend elbow in St.G.20 as just having served in a certain oil field for 48 days is investigated.The chemical composition, the mechanical properties, the metallographic structure and the energy dispersive X-ray spectrum of the bursting area of the said elbow are analyzed.As a result, it is identified that the cause for the failure is that the erosion gets the elbow pipe wall thickness reduced, thus the burst happens.Corrosive medium like H₂S, and CO₂contained in the conveyed medium strengthens the erosion effect.And the interactions by erosion and corrosion speed up the reduction of the pipe wall thickness.Finally when some location of the wall thickness of the elbow is reduced to a certain extent, local burst will take place due to the internal pressure of the pipe.