A three-span countryside P/C bridge has been found to have excessive deflections in two of the three spans. It is reported that the prestresses in tendons of two girders of different spans are lower than design values after investigations. The objectives of the present studies are (a) to prove that the change of dynamic characteristics of a bridge can be used as index for damage detection and (b) to study the possibility of application of system identification in defining damage quantitatively. It is found that the change of mode shapes can be used as an index for damage detection. In addition, the technique of system identification can define the damaged member in the P/C bridge successfully and quantitatively.