A design method of a fishway has been established on the basis of experimental hydraulics. However, a more rational design is enabled if a computational hydrodynamics, which has progressed rapidly in late years, is utilized. In this paper, a numerical fishway by three dimensional particle method is proposed to express two characteristics of fishway as follows: a three dimensional locally rapidly changing flow under complicated boundary condition of fins; and a free surface flow with a fragmentation of water, or a splash. As for a stream-type fishway, a vertical distribution of flow velocity calculated by the numerical fishway agrees well with results of previous hydraulic experiments. And a circulating flow cell in a transverse cross section is calculated for a further investigation of flow structure in a fishway.