Abstract: For huge complex structures, it is difficult to obtain enough information which is required necessarily in their global monitoring. Aiming at this problem, a Local Primary Frequency (LPF) method is proposed for a local damage identification using only a local measured response, which is easy to be performed in a real application. LPF belongs to the frequencies of a global structure, but the distortion of the corresponding mode mainly concentrates on a substructure under the local excitation and the subculture vibrates mainly in the form of this mode. In this way, the corresponding frequency is defined as the LPF of the substructure or it is said that the substructure has its LPF. Therefore LPF reflects more characteristics of the substructure and is sensitive to the local substructure damage, and hence only a Local Primary Frequency is enough to identify the substructure damage. However generally substructures don’t own their LPFs. Locating additional mass on the substructure is proposed as an efficient method to make the substructure have LPF. An experiment of a big space truss is used to verify the proposed method. Both the damage location and damage extent are identified accurately via the substructure LPF.