Civil infrastructure systems are prone to age related deterioration due to the exposure to aggressive environmental conditions and inadequate maintenance, and often causes reduction of their carrying capacities. Designing these systems for a particular service life and maintaining them in a safe condition during their entire service life have been recognized as very critical issues worldwide. There have been many damage examples of older steel bridge structures due to corrosion around the world during past few decades and they intensified the importance of attention to the careful evaluation of existing structures for the feasibility of current usage and strengthening them by retrofitting some selected corroded members to ensure the public safety. Therefore, a simple and accurate method to calculate the remaining yield and tensile strength by using a concept of representative effective thickness with correlation of initial thickness and the maximum corroded depth is proposed in this study, based on the results of many tensile coupon tests conducted on corroded specimens cut from a steel girder used for about hundred years with severe corrosion. The proposed methodology revealed more accurate and reliable estimation for the maintenance management of existing corroded steel structures.