The objective of the paper is to determine the structural dynamic characteristics from the low level structural seismic response data in small earthquakes. The off-line time-domain system identification method ARX (AutoRegressive with eXternal input) model is adopted to identify the time-invariant transfer functions, and then the structural parameters such as modal frequencies and damping ratios are determined. A 7-story reinforced concrete structure on which the strong motion monitoring system was installed and recorded seismic response in 3 small earthquakes was used for the analysis. It is found that the structural dynamic characteristics can be determined from low level acceleration response records in small earthquakes in which the structure can be assumed as a linear and time-invariant system. The results can be referenced by structure health monitoring, structural control and related works.