In this paper, free vibration behaviour of a W-shaped prestressed concrete girder is presented. The girder studied is a 24 m long prestressed composite box-girder with a cast in place slab at the top used in the viaducts of the Light Rail Transport System of the Greater City of Izmir in Turkey. The project is being carried by ABB- YAPI MERKEZI Construction and Industry Inc. Consortium. The dynamic characteristics of the girder have been evaluated theoretically using Rayleigh Principle. The microtremor measurements were carried out on the prestressed girder using vertical displacement sensors. The measurements were taken before a loading test on the girder in the elastic range and also after the loading test. From the data Fourier spectrum curves and as a result of these the first natural frequency of the girder and the related mode shape were determined. In this paper the results of the experimental and theoretical findings are discussed and presented.