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With increasing amount of pollution by thermal power plants in Industrial and developing countries, tend to use small-sized hydroelectric plants increased. In complex terrain regions there are usually a significant height difference between refineries and using place, the pressure can to produce electricity by power plants pressure reducer. The power plant is due to the relatively high initial cost less were used. Gradually, with the possibility of using pump as turbine and reducing the cost of building a micro power plant use the plant was expanded. Therefore, in this study centrifugal pump by CFturbo software was designed and for Numerical analysis of the three-dimensional fluid, the simulation was performed using the CFX software on SST k-ω turbulence model. The numerical results were compared with experimental in pump and turbine modes and showed good agreement. In order to increase the efficiency of the turbine pump (reverse pump), the decrease in the diameter of the impeller blades at different flow rates was investigated, which resulted was decrease in the amount of separation phenomenon around blades and causing increase in hydraulic quantities nearby the turbine bep point, but reducing the diameter at the flow rates very lower from bep point, didn’t have great impact at improvement of efficiency, at the bep point reducing the diameter, caused to increase 11.86 and 13.65 percent of the head and torque, and improved efficiency 1.26 percent.