Designing and producing the runner of Francis turbines which are employed extensively in hydroelectric power stations demands technical considerations and because of their complexities to make the blades of the runners which have complex three-dimensional surfaces is a very subtle activity. One of the important issues in this respect is to pay close attention to the hydrodynamic functions of the runner and its dependence upon the size and geometrical features of the blades. The present study considers the computer model of a runner made for the water power station of Dez dam which was made by Saya engineering company. This model serves for the production of a software environment employing fluent to simulate the fluid movement in the runner of Francis turbines. In this dissertation the results and primary calculations for simulation and for the sake of producing and arranging the software parameter. At the same time, the effect of the boundary conditions, the numerical mesh, the geometry of the output pipe which are all subject to the users choice have been systematically investigated and their results have been presented. Investigating the results of the conducted researches and performing numerical experiments have led to the production of a basic model with boundary conditions and software setup for simulating the flow in the runner of the Francis turbine. Based on empirical observations and investigations made on the dimensions of the blades constructed for the runner of Dez dam, various geometrical models have been produced in software. Each of these models has been made through producing a change in the dimensions of the blades of the runner. The features of theses models and their runners of production have been presented in this dissertation. Furthermore, the results obtained from numerical simulations of fluid flows in different geometrical models have been presented and analyzed. To investigate the hydrodynamic functions of the runner a few important parameters such as fluid flow, the torque exerted on the blades and finally the output power of the runner have been defined and the results obtained for these parameters have been presented and compared with one another. The investigation of the results reveals that the consistent increase in the width of the blades and the increase in the roughness of the surfaces of the runner would exert greater influence on the function of the runner and would make a noticeable decrease in the output of the runner. However, other geometrical changes such as the size of the welding seam, the temporary change in the width of the blade near the inlet and outlet edges and the existence of obliqueness on the surface of the blade in the domain of the investigated geometrical dimensions would not make any serious and significant changes in the hydrodynamic functions of the runner.