Abstract: Experimental studies are carried out for determining the dynamic characteristics and finding the cause of vibration during windy season of a 30m tall lamp post structure by using the measured responses of the structure to random ground motion caused by traffic and wind load respectively. It is found that a steady wind of Beaufort force 6 may cause vortex shedding of a frequency coinciding with the second bending frequency of the structure. The theoretical basis of random response to ground motion, the experiment method, data acquisition and analysis techniques for identifying modal shapes and modal parameters are also expounded.