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Abstract: An experimental study was conducted on two pairs of subassemblages of a typical 500kV transmission tower of the same type as those suffered the most severe damage during ice disaster in South China in 2008. The objectives are to study the failure mechanism of transmission towers under extreme load of freezing rain and to investigate the pertinent retrofitting strategy for increasing the load-carrying capacity of towers so as to prevent their collapse. The difference between specimens in each pair is that one had an additional diaphragm as measures of retrofitting while the other did not. The mechanical behavior, failure mode, strain and deformation at critical points, of the specimens were studied. The test results revealed that buckling of the main leg was the predominant failure mode of structures. For the two subassemblages without diaphragm, the out-of-plane deformations in the joints of diagonal bracings were relatively large and the buckled main angle members exhibited apparent torsion, which significantly decreased the load-carrying capacity of specimens. But for the two subassemblages with diaphragms, the out-of-plane deformations of cross-bracings were markedly inhibited by the added diaphragms and the buckling mode of the main member approached flexural buckling without torsion. As a result, the ultimate strength was increased by 18.3% and 17.6% for the single-panel and double-panel tower subassemblages respectively. It shows that the addition of the diaphragm significantly improved the mechanical performance of transmission towers by reducing the torsional effect on main members and inhibiting the out-of-plane deformation of diagonal braces. [Copyright &y& Elsevier]