We applied a molecular dynamics simulation technique to charged colloidal systems in order to investigate small ion distribution in thermal equilibrium and to evaluate the effective interaction among macroions. For a system containing a single macroion, screening by counter ions was evaluated. The screening becomes more complete for larger macroion charge. In spite of the large inhomogeneity, the chemical potential of small ions is constant for the entire system. Using a thermodynamic integral method, the effective potential (Helmholtz free energy) for two-macroion systems was evaluated as a function of macroion separation distance, and found to be repulsive under normal conditions. Many-body effect is also discussed by comparing the effective potentials for two-and three-macroion systems.

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