In offshore oil field development, we foresee an increase in a demand for oil well pumps having improved gas handling capacity. To meet the demand, by improving the performance of a radial-flow centrifugal pump of low specific speed, many kinds of impeller, fabricated by changing systematically design parameters such as outlet blade angle, location and shape of leading edge, number of blades, location of the split for dividing the blade length into two parts and outlet guide vanes, were experimentally investigated under air-water two-phase flow conditions to clarify their effects on pump performance. The optimum blade is found to be one that has a split in the latter half of the impeller and installed with an outlet blade angle of approximately 90°.