The fractal characterization in the evolving damages of concrete structures is studied based on physical model experiments. It is revealed that the surface-crack distribution of the damaged concrete structures takes on fractal behavior. This shows that fractal dimension is able to characterize the surface-crack distribution quantitatively. Then fractal geometry is employed to investigate the crack evolving process of concrete structures; and the relationships between the fractal dimension of surface-crack distribution and the mechanical properties of the damaged structures are discussed in detail. It is found that there exists a good linear correlation between the fractal dimension and the natural frequency

which shows that the fractal dimension of surface-crack distribution is able to act as a damage feature index to reflect the damage degree of concrete structures effectively. Compared with the conventional damage feature indices generated from vibration testing; this novel damage feature index is of some advantages and distinguishing features. It provides a promising approach to develop new technologies for structural health monitoring and safety evaluation.