In the macro scale level, a material point has its microstructure with many different orientations and grains in the mesoscale level. Material points respond to stimulus such as temperature and stress by changing their microstructure and topology in order to reach the minimum level of total energy. Grain growth caused by grain boundary migration is one of the responses. Nowadays prediction and control of the microstructure of materials are the main parts of developing advanced materials. The phase field method has become one of the most powerful methods for simulating material phenomena including grain growth. Grain growth can occur for many reasons, but normal grain growth occurs without any external stimulus and it occurs with the grain boundary migration to reduce grain boundary surfaces and consequently reduce grain boundary energy. Because of the high computational costs of simulating microstructural evolution, a representative volume element is considered as the representative of the whole domain. RVE size and boundary conditions have significant influences on the results of simulations. The purpose of this study is to investigate these influences based on the phase field method for normal grain growth. Several simulations considering both size and different boundary conditions including periodic, symmetry and microhard were performed for a polycrystalline metal and the optimum size for each boundary condition is obtained. Keywords: Simulation, Microstructure, Phase field, Grains, Grain Boundary, Grain growth, Polycrystalline metal