Abstract: The strain clamps of a 500 kV transmission line fractured during operation. The fracture causes of strain clamps were analyzed by means of chemical composition analysis, geometric dimension measurement, fracture analysis and mechanical simulation calculation. The results show that the root of the strain clamps was lconstrained locally, and the stress concentration occurred at the corner of the strain clamps when it is under large stress. The crack propagated under the action of alternating stress caused by wind force, and fatigue fracture occurred.