Abstract: An experimental investigation on swirl control in an S duct has been completed by the aid of a so called vortex reduced device (VRD) which is made of a metallic frame and placed upwards near the throat of the diffuser.Effects of geometric sizes of the VRD and its angle of attack on the flow in the duct were investigated in a wind tunnel.The flow parameters,including wall static pressure,total pressures and 3-D velocity field at the duct exit were measured.The optimum geometry shape and size (W id =0.0595) are found.The VRD with optimized geometry can effectively reduce the swirling range and improve the average total pressure coefficient at the exit.