The authors have developed a counter-rotating type hydroelectric unit where tandem runners counter-rotate inner and outer armatures in a peculiar generator, and the unit has penetrated into the market in the small/micro hydropower. The unit has promising advantages such as the rotational torque is counter-balanced itself between the front and the rear runners, namely the inner and the outer armatures. Such technology has been provided for the tidal power unit moored to the seabed with one cable. Tidal stream is abundant and clean energy resource whose power is predictable and sustainable. To generate power effectively from the stream, the unit must effectively work in response to change of the flow direction not only in day cycle but also in unsteady condition with turbulent fluctuation. This paper discusses the effects of the flow direction on the output of the tandem propellers by commercial Computational Fluid Dynamics (CFD) code.