Abstract: Molecular simulation is a powerful tool in studying properties of complex fluids composed of charged particles such as electrolyte solutions, room temperature ionic liquids and colloid solutions, where the long-range interactions play a determinative role. Several methods have been available for treating the long-range interactions between charged particles and point dipole. These include the Ewald sum (ES), the reaction field and particle-particle particle-mesh methods. Among these approaches, the ES is most commonly used. However, several ES versions have been occurred in the literatures of molecular simulation, and some times it is difficult for one to choose the right formula to use in molecular simulation study. The coulombic interaction energy between charged particles is divided into the sum of real space, reciprocal space and self energy, and theircal culation equations are obtained respectively using electrostatics theory and Fourier transformation method. The Ewald sum formulate have been derived and the clear physical picture involved has been depicted. A Monte Carlo computer simulation for electrostatic interaction energy of charged hard sphere system has been conducted at varying conditions, and a good agreement with MSA is obtained. On this basis the effects on the simulation accuracy and efficiency of real space cut distance r(cut), convergence parameter 1 and reciprocal maximum vector K(max) have been analyzed. From the theoretical and computer simulation presented here, the optimization parameters for rcut, 1 and K(max) are obtained as 0.5 L (i.e. half box length), 5.8 and 3～5, respectively.