In order to make clear the scale effects of the regenerative pump with non-radial vanes, the authors carry out an investigation of the pump performance using 18 kinds of impellers which combine six diameters from 94 mm to 180 mm with three vane angles 60°, 90° and 120° changing the dimensions of the flow channel and fluid viscosity. The following conclusions are obtainded : (1) The influences of impeller diameter on the performance vary with the vane angle. (2) The relations between pump elements and coefficients of shut-off head, flow-rate, torque and maximum value of internal efficiency are made clear. Consequently, the authors give the outline of design method considering the effect of fluid viscosity. To facilitate the description, two examples by the present design method are given. The calculated values agree well with experimental results.