在結構狀態評估的領域中，模態參數識別為一個很重要的課題。有鑒於土木結構之輸入反應不易得到，本論文探討頻率域分解法(Frequency Domain Decomposition, FDD)這個僅需要輸出反應之模態參數識別方法在土木結構狀態評估應用之可行性。藉由此直觀快速之方法，可依奇異值之峰值挑選來得到模態參數得到自然頻率、計算模態振形。文中以鋁梁實驗結合SAP 2000有限元素法，驗證頻率域分解法在以衝擊力進行結構模態分析之可行性。進一步計算得到結構柔度；引入橋梁狀態指標(Bridge Girder Condition Indicator, BGCI)的觀念，得到各模態之貢獻度(Modal Contribution Coefficient)。最後，將此方法應用在現地檢測中，求取橋梁之模態參數。 In the area of structures condition evaluation, modal parameter identification technique is very important task. However, it is difficult to measure the input loading data in the application of civil infrastructures. In the thesis, the Frequency Domain Decomposition (FDD) technique is applied to conduct the modal identification of output-only systems in the condition evaluation of civil structure. Using this visual and user friendly technique, we can find nature frequencies and mode shapes by simply picking the peak of singular value sketch. From the tests and numerical simulations on an aluminum beam, it shows that the FDD technique can also be applied to the structure system using impact force as the input loading. To establish a complete process of the system identification, the structural flexibility is calculated and the concept of Bridge Girder Condition Indicator is adopted to find the Modal Contribution Coefficient of different mode number. The capability of the techniques developed in this research to find the modal parameters of true structures was validated in field tests.