The objective of this thesis is to analyze a multistage submersible pump numerically, figure out the inefficient sections of flow and propose a modified design according to the simulation results. Hydraulic parts of a submersible pump are impeller, diffuser and return channel. In this study, the investigated pump’s stages have only impellers and return channels. According to the literature, the inefficiencies in pump stages might be caused by angle misalignment of impeller and guide vanes, channel geometries or vane designs. The investigated five stage submersible pump and its CAD models are provided by a manufacturing company. In the first part of the study, fluid domain is created for numerical computations and analyses are performed with ANSYS Fluent software. According to the simulation results, flow nonuniformities are observed in return channels and identified that the reason is guide vane geometry. The second part of the study includes the modified design of guide vane and set of simulations with different wrap angles to find out the optimized value. The simulation results of this thesis study are quite satisfactory when they are compared to experimental data of manufacturing company. After modifications, the nonuniformities which are observed in original design are diminished and efficiency of the pump is increased.