Recently, granules under vertical vibrations receive many attentions due to their importance in theoretical research and engineering application. In this paper, a two-dimensional Particle Image Velocimetry (PIV) system was used to examine the f/2 stripe pattern forming in a vertically vibrated granular layer. Since the PIV sampling frequency does not match with the vibrating frequency, a special identification-coupling method was adopted to combine the images taken in different cycles to offer the information in one complete cycle. The measured velocity vectors showed exactly the particle motions at various stages of a motion cycle, illustrating the alternating peaks and valleys on the layer top. Furthermore, quantitative results on the temporal evolution of velocity profiles were obtained and some other interesting phenomena were observed, such as the appearance of local structures (e.g. dual-phase layer structure) and the moving feature of the 'standing point'. The mechanism accounting for the occurrence of stripes on the surface is also discussed. This work will be of interest to a better understanding on pattern formation in the vibrating granular bed. / Singapore-MIT Alliance (SMA)