In compound open channels, the interaction between main channel and flood plain flow is the most important problem to be clarified. This interaction affects the resistance to flow and sedimentation on the flood plain. To carry on the research of the interaction, it is necessary to investigate the flow structure in the mixing zone between main channel and flood plain flow. In this paper, the three dimensional structure in the zone is visualized in horizontal, lateral and transverse cross section of the flow by the neutral buoyant tracer method and the hydrogen bubble method. The results show the existence of two kinds of vortex there, the one is the lateral vortex induced by the velocity difference of the two flows, and the other is the longitudinal vortex induced by the strong secondary flow which is observed near the external corner of the flood plain.