Centrifugal pumps are probably among the most often used machinery in industrial facilities as well as in common practice. After developing the computers and the CFD software, the numerical simulation of a centrifugal pump becomes very powerful tool to predict its performance. Nowadays using CFD technique to predict centrifugal pump performance save money and experimental efforts compared with the old techniques in centrifugal pump design. In the presented study a centrifugal pump with semi-open impeller has been tested experimentally in order to investigate its performance. These experimental results were also used to validate the numerical simulation. After doing these experimental tests, a 3-D numerical simulation of the same centrifugal pump with closed and semi-open impellers have been carried out using commercial software, ANSYS® CFX, to study and trying to enhance the performance of this centrifugal pump.