Abstract: A new method (non-convolution method) is introduced for determining the microstrains of polycrystalline materials by X-ray diffraction. The method is based on the principle of superposition of X-ray diffraction-line amplitudes/intensities from polycrystallites. It is considered that the contributions of the X-ray diffraction-line intensities from the size-strain crystallites could be separated. On the other hand, based on the Bragg law, it leads to the relationship between the angular deviations of diffraction X-ray lines from the size-strain crystallites. A series of expressions is therefore derived for determination of the size and microstrains of imperfect crystallites, for which the name direct determination method is proposed. The practicability and reliability are evidenced by the determination of as example MgO-fiber microstrains presented.