The heteroepitaxy of KCl on a KBr(001) substrate, with a lattice mismatch of -4.7%, was studied, and the relaxation mechanism of the lateral lattice constant was clarified. The stable atomic configurations were calculated by minimizing the total interaction energy between the ions. At submonolayer coverages, two-dimensional island configurations are more stable than three-dimensional structures, indicating a layer growth mode. At integer monolayer thicknesses, both the strained layer structure with the lateral lattice constant of KBr and the pseudomorphic commensurate layer structure with KBr(001) were investigated. The strained layer structure is more stable than the pseudomorphic structure below the critical thickness of three monolayers. The commensurate layer separates into two regions : a strained layer region and a dislocation region with one extra cation and anion row relative to the KBr substrate. The width of the dislocation region increases as the layer thickness increases, corresponding to the observed gradual relaxation of the lateral lattice constant above the critical thickness.