It was supposed that steel bars in concrete begin to corrode when concrete cover is entirely carbonized, and the time of corrosion initiation was calculated by carbonation model of concrete under stress proposed through test study. Diffusion coefficient of O₂ was educed with diffusion coefficient of CO₂ in concrete under stress. The effect of stress on corrosion rate of the steel bar was confirmed by experiment study.Based on the theory of corrosion electrochemistry, prediction model for corrosion of steel bars in concrete under stress before corrosion cracking was presented. The computation model was well verified by a practical engineering project. Significantly severer reinforcement was observed in higher stress concrete zone. It can be contributed to earlier corrosion initiation time and higher cor-rosion rate for steel bars in concrete under stress.