Field measurement was conducted on the evaluation of dynamic amplification factors (DAF) for four existing T-girder bridges. Both ambient vibration testing and vehicle impact testing were carried out on the bridges. Ambient vibration testing is relatively easier to conduct and can provide detailed vibrating information of the structure. However vehicle impact testing is indispensable to obtain the impact factor of the traffic load. The measured vibration frequencies matched well to those of calculated values. This means that the finite element model may enable good predictions of the actual behavior of the bridge. The measured DAF for these bridges located in the interval [1.05, 1.22].