Vacancy interaction in crystalline Si wafer is studied by numerical simulation based on the Tight-Binding Molecular Dynamics (TBMD) method. We evaluate an effective interaction between vacancy pair in bulk and in a thin slab as model systems of a Si wafer. In the case of vacancy pair in bulk crystal, results reveal that there is an effective “capture radius” of the vacancies to form a divacancy at around 8Å at 300K and 13Å at 600K. Surface structure involving vacancies in Si slab undergoes a reconstruction accompanying dimer-rows in [110] direction initiated by the vacancies. The capture radius is also found to exist in the slab and binding energy of the divacancy has depth dependence due to the surface effect.