In actual open channel flow, secondary flow is always generated near the side wall even in straight channel and this seems to influence the flow structure. In this study. secondary flow and turbulent structure in a rectangular open channel flow were investigated experimentally. As the results. secondary flow consists of the free-surface vortex which includes the horizontal flow from the side wall to the center of channel and the bed-wall vortex which pairs with the free surface vortex. These vortices interact with the primary flow and dominate three dimensional flow structure. The differences between open and closed channel flows are remarkable and therefore the effect of free surface plays an important role on the flow structure in open channel flow.