Corrosion of steel reinforcement is spatially distributed over RC structures due to several factors such as different environmental exposure, concrete quality and cover. Ignoring the effect of spatial variability is a drastic simplification for the prediction of the remaining service life of RC structures. Therefore, it is essential to identify the parameters influencing the spatial steel corrosion and structural performance of corroded RC structures. In this paper, an experimental research was conducted to study the effects of current density, concrete cover, rebar diameter, and fly ash on the spatial variability of steel weight loss, corrosion crack, and structural behavior of corroded RC beams using X-ray and digital image processing technique. The test results showed that low current density induced highly non-uniform corrosion associated with few large pits and cracks at certain locations while higher current density produced more uniform corrosion and cracks occurred over the whole beam. Gumbel distribution parameters were derived from the experimental data of steel weight loss to model spatial steel corrosion. A novel approach was established to assess the reliability of RC structures using finite element analysis and probabilistic simulation considering the spatial variability in steel weight loss. Using the Gumbel distribution parameters derived from the steel weight loss data associated with higher current density may underestimate the non-uniformity of corrosion distribution which can lead to an overestimation of the load capacity of corroded RC structures.