A new model of repaired concrete which divides the bonding interface into a penetrating layer, a strongly-affected layer and a weakly-affected layer was put forward. The model is mainly based on the observation of the microstructure of interface between fresh and old (3 months to 60 years ) concretes by using scanning electron microscopy. Then, the mechanism of the microstructure formed was analyzed. Finally, the relationship between the micro-structure and macro-mechanical performance of the interface was discussed.