The machine Reynolds number and the specific speed of centrifugal blood pumps are lower than conventional pumps, therefore, it is difficult to apply a general design method to such centrifugal blood pumps. In this research, the influence that each part shape exerted on adult and pediatric blood pump performance were examined, moreover, the method of the design of pump that is appropriate for low specific speed and low mechanical Reynolds number pump were xamined. As a result, the efficiency of these pumps can be estimated using the specific speed considering viscosity.