Mostly recent, pump operating is dealt with a problem of value in air-water two phase flow. However, it is difficult to examine a cause of performance slow down closely due to entrained air. This paper tries to analyze the single-phase flow and air-water two-phase flow characteristics according to semi-open type impellers with 3, 5, 7 blades through experimental apparatus. The casing is made up with transparency acrylic for confirmation the flow pattern. The pump rpm and the shaft torque are measured by rpm sensor and torque sensor. The test pump's maximum rpm, head, kw are 1750, 15m, and 1.5kw, respectively. The reappearance and reliance of apparatus are satisfied through an verifiable experimentation with separate ways. As a result in experiment, characteristics of single stage centrifugal pump are excellent in the order of an impeller with 7, 5, 3 blades in a single-phase flow. But the break down of pump has been happened slowly in the order of an impeller with 3, 5, 7 blades in air-water two-phase flow. Also, this paper tries to analyze flow pattern between impeller and casing in order to examine a cause of break down in air-water two-phase flow.