Most of the engineering fields, including mechatronics, require reliable operation of structures. Because of that, structural health monitoring (SHM) is rapidly becoming an integral part of critical structures. There are many different SHM techniques and each of them has its own advantages and disadvantages related to techniques sensitivity to defects and operational costs. A lot of study is done in vibration based SHM, especially in modal analysis and operational modal analysis (OMA) fields. This paper proposes signal processing algorithm called Modal Field Comparison Method (MFCM) for OMA based SHM system. The method considers processing of vibration responses, such as acceleration or deformation signals, received from multiple vibration sensors installed on the structure. MFCM gives a single value estimation of how the condition of a structure has changed in relation to reference condition. Theoretical basis of the method is presented, as well as results of experimental study on a cantilever beam with 12 deformation sensors (piezo films). Sensitivity to condition modification was also studied by installing test masses on the beam. The paper discusses good and promising results of the study, but further work is necessary to establish foundation for a potential automatic SHM system.