This paper presents a novel approach for optimizing and also improving a flash-binary geothermal power plant whose binary cycle is an organic Rankine cycle (ORC) which is using various combinations of zeotropic mixtures as working fluid. All of the obtained results are optimized with particle swarm optimization (PSO) method for maximum total output power which is the objective function of the problem. First, the optimization is performed in the certain amounts of mixtures' mass fractions. Then, the optimal values for the mass fractions are found. The results indicate that Pentane containing combinations show better performances. For instance, when the ORC unit is using the mixture of Pentane(0.45)/Butane(0.55) the highest output power is gaining: 1376.87 (kW) from the ORC unit and 5726.44 (kW) from the whole system. Also, the highest improvements in utilization of zeotropic mixture instead of pure fluids are obtained by this mixture which are 18.769 (%) with respect to ORC's output power and 3.950 (%) with respect to total output power. Finally, an investigation on flash chamber pressure effect on the system performance is accomplished and the results reveal that with increasing the pressure, the total output power decreases. Although lower flash chamber pressure seems to be a suitable choice, the investigation on the size parameters (SP) of the turbines shows that it is better to choose a mean amount of pressure for the flash chamber, thereby having the affordable amounts for the size parameters and also obtaining an adequate amount of total output power.