Abstract: The modal parameter identification of a structure using the dynamic response signals under ambient excitation is an important step for a finite element model updating and condition assessment. Up to now, it is still difficult to obtain reliable identification results, especially for damping parameters. Firstly, the approaches to identify modal parameters using eigensystem realization algorithm (ERA) in the cases of single-input single- outputs (SISO) and single-input multiple-outputs (SIMO) are programmed via MATLAB respectively. Secondly, modal frequencies, modal damping ratios and mode shapes of a rigid frame-continuous girder bridge are identified based on the dynamic test results under ambient excitation. Finally, modal parameters of this bridge are also identified by the stochastic subspace identification (SSI) method embedded in a commercial soft. The comparison of the results indicates that identified modal frequencies and shapes from ERA and SSI agree well. But there are obvious differences in the results of damping ratios. Thusly, it is still difficult to confirm the precision of the identified modal damping ratios.