This paper is intended to discuss about vibration monitoring and a field experiment carried out on an actual steel plate girder bridge. The aim of the monitoring and field experiment is to identify the dynamic parameters of the bridge before and after applying artificial damage. Fatigue cracks propagated from the base plate of bearings to the web plate were considered as the artificial damage. The bridge was instrumented with accelerometers at a number of locations on the lower flange of the bridge to record vertical vibrations. The bridge was excited via moving vehicle tests and the resulting acceleration signals were used to identify dynamic parameters, such as the bridge mode shape, natural frequency and damping constant. For structural analysis of the bridge, a finite element model of the bridge was created whose structural properties and dimensions were decided based on the design documents and measurements during the field experiment. In numerical analysis, the artificial damage is modeled by assuming the pseudo crack. The frequency, which is one of the typical modal parameters, was adopted as the damage sensitive feature to monitor changes in the damage sensitive feature due to the artificial damage. Observations demonstrated that the frequencies identified from the field experiment were varied according to the bridge condition.