In compound channels, in addition to shear flow originated from the bed (boundary layer flow), other forces are generated by momentum transform between the main channel and the floodplain (free shear layer). Due to such special type of momentum transport, a complicated three-dimensional flow structure forms in a compound channel. Previous studies showed that in a compound channel, secondary currents are enhanced for shallow overbank flow and consequently the complexity of flow structure increases. However, this complexity has not be described properly. To explore turbulent structure of a shallow overbank flow, flow field is measured in a compound channel with vertical walls using Particle Image Velocimetry. The results show that in the main channel, the maximum amount of streamwise velocity occurs below the floodplain level. Whereas in previous studies in compound channels with inclined transitional wall, turbulence intensities profiles in the main channel showed two different trends at lower and higher elevations of the floodplain invert, in the present study three different increasing or decreasing trends were observed for Reynolds shear stress and longitudinal turbulence intensity profiles and four different trends was observed for vertical turbulence intensity. Bed shear velocity was approximately constant in the floodplain but it increased near the interaction zone.