The fracture reason of a strain clamp in 500 kV transmission line is investigated by macroscopic examination，tensile testing，metallographic examination，scanning electron microscopy and energy spectrum analysis． Ｒesults show that mechanical properties，microstructure and chemical composition of the steel core and aluminum tube are qualified and can satisfy the requirements of relevant standards． Ｒepeated crimping trace and obvious bending deformation can be clearly observed on the surface of steel anchor，resulting in larger stress concentration and the extrusion friction damage of steel core． After long time running，the damage of steel core is aggravating and steel cores suffer the fracture successively，which leads to the fracture of external aluminum tube． Thus，the accident happens．