To prevent efficiency loss owing to the collision of water flow with the main shaft, atmospheric air is naturally sucked in through a hole in the casing of a cross-flow turbine using the negative pressure in the runner chamber. In this study, perfor-mance changes owing to the flow rate of suction air were investigated under two different effective head conditions. The results show that if Q_air/Q_w was greater than 0.1, the performance, such as efficiency based on runner height and runner chamber pressure, was almost the same even if the air flowrate was further increased. Assuming the draft tube outlet is 2.5m below the runner center, which is used as the standard height for effective head, efficiency decreases as the suction air flowrate increases. Comparing at the same Q_air/Q_w, the decrease in efficiency is smaller when the net positive suction head (NPSHa) is lower because the bubbles become denser when compressed while flowing downward. However, when NPSHa is high and the effective head is low, the slow flow velocity in the pipe causes bubbles to aggregate. This increases buoyancy and raises the void fraction, thus reducing efficiency.