An experimental study was carried out to develop a new type of two-way diffuser Suitable Cor a fluid flow energy conversion system. It is known that the power available from the fluid flow is proportional to the cube of the free stream velocity of the flow. Therefore, in order to take higher power Output from the fluid flow, it is very important to construct a suitable system to increase the flow velocity. For a wind turbine, it has been reported that the speed of wind passing through it is dramatically increased by the use of a diffuser with a brim around the turbine. In this Study, a new type of two-way diffuser suitable for a flow periodically changing its direction, such Lis a tidal current, was developed, applying the system to accelerate the wind speed for the wind turbine. The effects of the brim height and the outside body shape on the diffuser performance were experimentally investigated by measurement of the pressure and the velocity distributions along the center axis of the diffuser and around the diffuser. The present study is the first one to clarify the effect of these on diffuser performance. (C) 2004 Elsevier Ltd. All rights reserved.