The centrifugal pumps are widely used in the oil industry and play a vital role. Thus, performance optimization is very important in the centrifugal pumps. In this paper, 3-D flow of centrifugal ESP pump has been numerically simulated. This study aimed to investigate the geometrical variation effect on the performance and flow pattern inside one stage of the pump. For this purpose, the effect of distance and angle between hub and shroud at passage outlet of impeller on pump performance has been assessed. The presented model is centrifugal pump with 3500 rpm with six blades for impeller and eight blades for diffuser. The ANSYS-CFX software has been used in this investigation. In order to modeling the impeller and diffuser, movement the multi reference frame and for interconnection of them the frozen rotor is used. Furthermore, the K-ω sst turbulence model is applied. The results of simulation are well-adapted with experimental results. The result of this study disclosed that by increasing 8.25 degree of the angle between hub and shroud, the amount of head and efficiency rise up in the pump. It also shows that the optimum distance between hub and shroud by considering a goal of simultaneous maximizing of head and efficiency is 1 mm. Variation of two parameters of one stage of pump, simultaneously, cause to increase head by 7.6% and efficiency by 1.65%. These variations have been occurred due to the separation region of flow decrease on the surface of the diffuser blades.