The triple axis mode X-ray diffraction method is described, which has been used to measure the spatial variation in the lattice parameters of {1 0 0} lightly Cr-doped SI-GaAs laser windows. It has been found that a remarkable spatial variation of elastic strain exists in the window wafer. The results show that the thinner the wafer is, the smaller the stress spatial variation in the wafer. The elastic strain in the window wafer can be released by thermal annealing. The method can also be applicable to the precise lattice parameter measurements of the compound materials. (C) 2003 Elsevier Science Ltd. All rights reserved.