A deconvolution method for the X-ray diffraction line profile is proposed, which is based on the conventional least-squares method. The true profile is assumed to be a functional form. The numerical values of parameters of the function assumed are determined so that the calculated profile, which is a convolution of the function and the instrumental profile, has a minimum deviation from the observed one. The method is illustrated by analysis of the X-ray powder diffraction profile of sodium chloride 222 reflexion under the assumption that the true profile is a Gaussian or a Cauchy function.