Entrained air in a centrifugal pump has undesirable effects on the pump operation and knowledges of the pump performance under air admitting conditions will be needed for actual purpose. In order to investigate the performance, a semi-open impeller pump with a transparent casing was employed and the behavior of the entrained air bubbles in the pump was observed. The impeller work and hydraulic loss of the pump, together with energy loss for air delivering are discussed. The total head of the pump is decreased by the air admission, but the head developed by the impeller remains substantially constant, The drop of the pump head is mainly caused by the energy consumption for air delivering within the range of q_s/Q≤0.02, and when q_s/Q is increased beyond this range the hydraulic loss due to the flow resistance of air bubbles increases its weight in the head drop. These relations are given in empirical formulas.