A series of two-phase pump performance tests were carried out by using three model pumps: two high head (225 m) pumps which were similar in shape but different in size, and one low head (25 m) pump. Experimental data were reduced with two-phase mixture density at exit of impeller. The results showed that pump heads for volumetric quality less than 10% agreed well with those in single-phase flow. This result implied that the two-phase flow in impeller was homogeneous in this range. On the other hand, large degradations of pump head were observed and separated flow was suggested when volumetric quality was larger than 10%. The degraded heads of the model pumps were compared with analytical results derived from a separated two-phase flow model. The calculated results were within the data scatter.