The unique features offered by wireless smart sensor networks (WSSN), including high integration, wireless communication, on-board computation, small in size, simple installation, and cost effectiveness, facilitate a new paradigm to structural health monitoring (SHM) for civil infrastructure. The paper is an example use of WSSN and the Illinois SHM Project toolsuite. An experimental study of modal analysis of a simply supported plate structure has been undertaken. The eigensystem realization algorithm (ERA) in conjunction with natural excitation technique (NExT) and stabilization diagram (SD) is used to extract the modal information, including natural frequency, damping ratio and mode shapes, from the ambient vibration test data. The results show that WSSN designed for SHM application utilizing a smart sensor platform suited to high-frequency, high-fidelity data acquisition and demanding data processing algorithms.