Steel structures are usually subjected to simultaneous corrosion and service loads which are cyclic in nature. This paper proposes a new methodology for prediction of fatigue failure of steel structures subjected to simultaneous corrosion and fatigue stresses. A model is developed to determine the effect of corrosion on S-N curve of steel subjected to corrosion and fatigue and verified with experimental results. The fatigue damage to steel is modelled as a stochastic process allowing for corrosion effects on both stress range and S-N curve. The first-passage probability method is employed to determine the time-dependent probability of fatigue failure of the steel structure subjected to corrosion and fatigue stresses. A worked example is provided to demonstrate the application of the proposed methodology. It is found in the paper that neglecting corrosion effect on S-N curve can lead to 90.43% reduction of the probability of fatigue failure of steel structures subjected to corrosion and fatigue stresses in 120 years, which can be catastrophic. The methodology proposed in this paper can help structural engineers and asset managers to predict the fatigue failure of steel structures when they are subjected to simultaneous corrosion and fatigue stresses.