Dynamic analysis of the civil structures such as bridges, towers and buildings is required for their design and maintenance. Modal analysis is a powerful tool to conduct some part of dynamic analysis in determination of the modal parameter in terms of natural frequencies, damping factors and mode shapes. However, excitation of these structures is usually difficult and sometimes impossible. As these structures are usually excited by ambient forces such as wind, this idea is suggested that the structure is modeled considering the natural forces as the inputs.However, the ambient forces are unknown and have a complicated nature to be measured. An alternative approach is using the operational modal analysis concepts in which only the responses are measured and the modal parameter are extracted. In this article Frequency Domain Decomposition (FDD) method is used for identification of the modal parameter of a clamped-clamped beam and the results are compared with those of the FEM. The operational modal analysis is conducted on a type of a bridge under ambient forces in a real test and the results are compared with those of the conventional Modal testing. The results confirm the method for engineering applications.