The vibration characteristics of structures have been considered to be one of the important evaluation indices to clarify the seismic performance. This paper presented the results that were obtained from the ambient vibration of three existing school buildings before and after measurements of seismic retrofitting using a wireless measurement system. Results and comparisons between the modal parameters, first natural periods and story stiffness obtained by the before and after measurements of seismic retrofitting demonstrated that the measurement and data processing techniques used are valid for evaluating the seismic retrofitting works.
