The least square method is an essential method to compute lattice parameters with high precision. The Cohen’s least square method with or without a corrective function of diffraction angle is discussed. The normal equations of seven crystal systems, formulas to compute lattice parameters and estimate error are given. The modified Levenberg-Marquardt iteration algorithm, which is commonly applicable, with wide convergent range and fast convergent speed to compute the lattice parameters and corrective function parameters, is also given. As examples, the lattice parameters of several crystals belonging to the different crystal systems were computed，and the results showed that the least square method is a method to obtain accurate lattice parameters, and the parameters of the corrective function ChuG was computed with the modified Levenberg-Marquardt method, which shows that the method is effective to compute the corrective function parameters.