Existing seismic design and assessment documents do not contain detailed specific guidance on seismic performance assessment of existing reinforced concrete (RC) structures, which have suffered from corrosion. This may cause unsafe consideration of reinforcement corrosion and, consequently, misleading seismic evaluation of such structures. In this study, six columns were subjected to different levels of accelerated corrosion process (causing different levels of damage, from slight to severe) and then were tested under constant axial and reversed cyclic lateral loading. The original aspect of the study is the clear demonstration of the adverse effect of corrosion on drift capacity of the columns. Furthermore, based on the test results, a method is proposed for calculating the deformation capacity of RC columns as a function of corrosion level for reliable seismic safety evaluation.