A flag flapping in the wind is a classical fluid-structure interaction problem that concerns the interaction of elastic bodies with ambient fluid. The fluid-flag interaction can give rise to three self-sustained oscillation states under certain conditions, i.e., stretched-straight state, periodic state, and chaotic state. This paper reports an experimental study of a cantilevered polydimethylsiloxane (PDMS) flag flapping in a uniform flow at a periodic state. A heavy flag is well designed, with metal strips imbedded in the PDMS sheet. Immersing the elastic but self-sustaining flag into the water flow, we use the time-resolved particle image velocimetry (PIV) and visualization techniques to obtain the whole flow field around the midspan of the flapping flag. A unique PIV image processing method is used to measure the near-wall flow velocities around the flap-ping elastic flag at the periodic state. The image processing technique adopts a radon transform technology to determine the moving interface in the particle images. The interface velocity distribution is subsequently calculated. Artificial particles of uniform size with the interface velocity are added into the flag region. Therefore, the whole velocity field over a flapping period is accurately obtained, giving the basic data to analyze the flag flapping. It is found that there exists an inflection point in the envelope curve of the flag flapping. Based on the analyses of the flapping states and velocity fields, a unified flapping Strouhal number (St = 2Af/U) is proposed by choosing the amplitude of the inflection point as the characteristic length, which is similar to the Strouhal number of the flow around a circular cylinder over the same range of Reynolds number.