The aim of present study is to perform a numerical work to investigate flow inside a semi axial centrifugal pump. The results of the study can be used to improve the design of the pump.The Navier Stokes equations with appropriate boundary conditions are solved for a 3 dimensional rotating geometry. To solve the governing equations, Fluent program is used. Fluent is a commercial CFD program, which has been developed based on the finite volume method. The mesh for the flow volume is created by Gambit.The k -e turbulence model is used to handle the turbulence inside the flow. Obtained results are compared with experimental test results. An acceptable agreement between the numerical and experimental results is observed.Based on the obtained results, the velocity vectors, pressure distributions on impeller and diffuser blades and flow patterns are plotted for three different flow rates.The necessary discussions are performed for these results. It is observed that the design of the impeller provides a regular flow inside the channel; however some vortexes are observed in the channel between the diffuser blades.The effect of the surface roughness is also investigated and the problem is solved for four different roughness values (0, 50, 100 and 250 m). It is found that the roughness of the surface affects the characteristic curves of the pump. By increasing the roughness, the head and efficiency of the pump decrease however the consumed power is almost constant.