Intensive research efforts have been spent on vibration-based structural condition monitoring since structural vibration parameters are closely related to structural stiffness which can be used to identify, locate and quantify structural damage. This makes the vibration-based methods quite attractive for structural condition monitoring. This paper proposes to use strain mode for structural damage identification. A new index named Breach Threshold Value of Damage Levels (BTVDL) is proposed to quantify damage. To demonstrate the proposed method, responses of a circular arch with single and multiple damages are simulated using commercial software ANSYS. The simulated dynamic responses are used to identify the location and degree of the damage of the arch. The results demonstrate that strain mode is more sensitive to damages than others indexes such as frequencies and curvature modes. To further verify the method, a scaled arch model was fabricated and tested in the laboratory. Experimental results also confirm the high sensitivity and accuracy of using strain mode in structural damage identification. It is found that the simulated damages can be reliably identified by using the first two strain modes. The proposed method is more sensitive to damages and can be applied to monitoring structural conditions.