An expanded mode method constructed based on the modal analysis method and an energy approach is presented for size identification of a crack with given location in a damaged structure. Measured natural frequencies and mode shapes of the structure with or without a crack are used to compute the strain energies of the structures subject to free vibration via a quasi-static approach. The two types of strain energy are then combined with the work required for the crack formation to construct the energy balance equation from which the size of the crack is evaluated through an iteration procedure. The theory of fracture mechanics is used to derive the work required for crack formation. Examples of the identification of crack sizes for a number of damaged beam structures are given to illustrate the applications of the proposed expanded mode method. Factors affecting the accuracy of crack size identification are investigated using a cracked cantilever beam as an example. The results show that the present method is promising for practical applications.