A new physical parameter is presented and it is applied to damage detection to address the two main challenges in the field of vibration-based structural health monitoring: the sensitivity of detection and the requirement of data of the baseline state. The parameter is also shown to be not affected by noise in the detection ambience. Assuming the damaged structure to be a linear system, its response can be expressed as the summation of the responses due to the undamaged and the damaged part. If the part of the response due to the damage is isolated, it forms what can be regarded as the damage signature. In this paper, the occurrence of damage signature is investigated when the damaged structure is excited at one of its natural frequencies, and it is called partial-mode contribution. The existence of damage signature as partial-mode contribution is first verified using an analytical derivation. Thereupon, its existence is ascertained using finite element models and by doing experiments. The limits of size of the damage that can be determined using the method are also investigated.