In recent years, Smart Grid Technology (SGT) has become the next generation of communication and control techniques in power grids, which provides bi-directional communication and control of power flow to enhance the power supply capability and reliability to a power network. One of the most important competent of SGT is the Distributed Generations (DGs) that are interconnected into the Distribution System (DS) to incorporate renewable energy sources into the power grid, to reduce power losses in transmission, and to improve grid voltage quality or stability. In fact, installing DGs might cause negative impacts on the grid, such as increasing power loss, operation costs, and poor voltage profile. Hence, proper design of DS, including optimal siting and sizing the DGs at appropriate location and with proper capacity play an important role in SGT. This work aims at finding the optimal locations and sizes of DGs in the power grid, using the IEEE 57-bus network as a test case. The tasks are accomplished by formulating an optimization problem for minimizing the active power losses under acceptable grid voltage variation and solving the Global Optimization (GO) problem using Metamodel-Based GO method. The power network model are built in MatPower, and the network simulation and optimization are carried in MATLAB with MatPower interface. Results for the modeling, simulation and optimization are presented to show the benefit of the proposed method.