Optimal Power Flow (OPF) is one of the main functions of Power Generation Operation and Control. It determines the optimal settings of generating units, bus voltage, and transformer tap and shunt elements in Power System with the objective of minimizing total production costs or losses while the system is operating within its security limits. The aim of this paper is to propose a methodology (GA-Matpower-OPF) that solves OPF including both active and reactive power dispatches. It is based on combining the Genetic Algorithm (GA) to obtain a near global solution, and the package of Matlab m-files for solving power flow and optimal power flow problems (Matpower) to determine the optimal global solution. This method was tested on the modified IEEE 57 bus test system. The results obtained by this method are compared with those obtained with GA or Matpower separately.