In this paper, the strain aging of cold worked structural steel is studied to determine allowable cold working radius of steel bridges. In Japanese bridge design code, the allowable cold working radius of steel plate used in bridges is specified to be more than 15 times of thickness of steel plate (This corressponds to the plastic strain of 3%). The effects of strain aging on modern steels are discussed based on the experimental data of Charpy absorbed energy. It was found that the plastic strain of 7.5%-10% is acceptable to ensure the required toughness of structural steels. As a result, the change of allowable cold working radius is proposed with regards to the level of Charpy absorbed energy.