The dissociative adsorption of a hydrogen molecule on the nickel(100) surface with point defects is investigated using the embedded-atom method (EAM). The potential-energy surfaces (PES) for H₂ dissociation on both perfect and imperfect Ni(100) surfaces are presented, based on total-energy calculations. it is clearly shown that as the H₂ approaches the Ni(100) surface along the entrance channel, the H-H bond is progressively weakened while the H-metal bonds begin to form; finally the H₂ is adsorbed on the surface in the form of two independent H atoms. This dissociation process is affected by the vacancy and impurity atoms existing in the Ni substrate. The activation barriers (E_a) for the dissociation of H₂ through various pathways are calculated. The barriers for the dissociation of H₂ on the perfect Ni(100) surface are found to be low (about 0.08-0.09 eV. corresponding to different dissociation pathways). The existence of vacancies enhances the dissociation of H₂ by lowering the activation barrier height and providing more adsorption sites. However, the impurity atoms (Cu, Pd) can impede the dissociation of H₂ on the Ni(100) surface by increasing the activation barrier height. The adsorption heat of H₂ chemisorption on the contaminated Ni(100) surface is also calculated. It is found that the effects of impurities on the dissociation of H₂ vary with the dissociation pathways and the impurity sites.