A simple and e!ective procedure for conducting the free vibration test on highway bridges is presented. The impulsive force in each direction is generated by a loaded truck that either stops suddenly or falls down from a rigid block. The feasibility of the procedure is demonstrated in identi"cation of the dynamic properties, i.e. the vibration frequencies, mode shapes, and damping ratios, of a three-span box-girder concrete bridge using the Ibrahim Time-Domain (ITD) technique. Up to 14 modes have been identi"ed for the present case. For the purpose of veri"cation, ambient vibration tests were also carried out, with the data processed by the random decrement (Randomdec) technique to yield the free vibration response, followed by the ITD technique. The dynamic properties identi"ed from the two types of test correlate very well with each other, indicating the validity of each procedure described herein. Although many more modes can be identi"ed from the free vibration test because of the higher quality of data produced, the easiness and general applicability of the ambient vibration test can still be appreciated. A comparison of the experimental results with those by the "nite element method indicated a lesser degree of correlation, implying that the "nite element model adopted in design requires further re"nement, say, through a more realistic evaluation of the boundary conditions, geometric and material properties of the bridge.