Abstract: Dividing Distribution Function (DDF) method is one of the methods by which the particle size distribution of ultrafine powder can be evaluated from its small-angle X-ray scattering data. In this paper, the stability of the solution obtained from DDF method has been studied through optimizing the coefficient matrix, introducing a damping factor and a least square treatment. All the calculations were accomplished with a computer. It was shown that the average deviation of the size distribution obtained is not greater than the error of the scattering intensity measured as long as the corresponding requirements are met.