A new least-squares method is described, which can be applied to the determination of lattice parameters in cubic, tetragonal, orthorhombic and hexagonal crystals. This procedure is constructed by a combination of the Cohen’s analytical method and a method proposed by Lutts, being characteristic of using any pair of diffraction lines. In order to eliminate partially the influence of the systematic errors, a modified observation equation is used effectively in the present method. The following advantages are considered:(1) The number of “unknown parameters” decreases by one in comparison with the Cohen’s method,(2) observation equations greatly increase in number to be adequate in the least-squares treatment, and(3) for cubic crystals, it is not necessary to use the least-squares method.Thus, the present method allows us to calculate the lattice parameters more easily and rapidly than the Cohen’s method.The lattice parameters of a Pb-15at%Bi alloy (cubic) and a pure In (tetragonal) were determined by the present method. For orthorhombic crystals, a comparison of procedures was made. The results were found to be almost the same as those by Cohen’s method.