The effect of impeller side clearance (the gap between the impeller face and the casing) on the centrifugal pump performance has been investigated numerically through monitoring the change of the pump head and efficiency at different flowrate. 3-D numerical simulation has been carried out using commercial software, ANSYS® CFX. The numerical simulation has been compared with experimental results to verify the numerical solution. In addition, the results have been compared with the closed impeller with front shroud at the same operating conditions It has been shown that the impeller side clearance have a great regression effect on the centrifugal pump performance. This regression on performance could be attributed to the drop in the pressure difference between the impeller inlet and outlet. And the secondary flow across the blade tip which impedes the core flow.