In this paper, it proceed the fascinating experimental study to use the system identification(SI) for a long time measurement. The aim of the study is to identify the dynamic characteristics using the SI based on ambient vibration(AV) tests for monitoring systems at building structures. White noise, which is naturally occurring for harmonic components, and free vibration tests are employed in obtaining output data. A definite answer of the dynamic characteristics manages to extract from the frequency domain decomposition(FDD) method under the condition of the white noise. The singular value of the following test is used by FDD. These results show the validity of AV test and the usability of FDD in comparison with two tests. The result of the study is to design practical criteria for SI for building structures based on AV tests.