Air-water two-phase flow performance of conventional centrifugal pumps causes the impermissible head deterioration even at an inlet void fraction less than about 10%. A tandem arrangement of double rotating cascades and higher blade outlet angle more than 90° only in outer rotating cascade has been proposed as a centrifugal pump impeller with high performance in air-water two-phase flow condition. To obtain further improvement of pump performances, a diffuser cascade is installed downstream of impeller outlet. In design of air-water two-phase flow centrifugal pump in various size and operating conditions as well as in single-phase flow, similarity law of pump performances is very useful. The similarity law of impeller diameter, blade height and rotational speed is investigated for the proposed impeller in the present paper. As the results, the similarity law of impeller diameter and rotational speed is certified experimentally even in two-phase flow condition. In addition, influences of blade height on air-water two-phase flow performances indicate a little difference from the similarity law. This difference is, then, discussed by using the results of static head on the shroud wall and air behaviors in the impellers.