Based on a study of the X-ray optics of the monochromator, of Guinier- type focusing camera, the important role of this kind of camera in improving the resolving power for X-ray diffraction patterns is analysed. It is considered that the improvement in resolution was resulted from the following considera- tions;(1)the monochromator is made to reflect only the a1-component of the Ka doublet;(2)the efficient radius is much increased and the spectral line widths are much reduced by using of the focusing camera; and( 3)the effect of combination of Ka doublet污due to the arrangement of the moncchromator corresponding to the focusing camera in the form of the(+，一)model, based on the conception of double crystal diffraction. Experiments showed that Guinier focusing monochromator camera is chasa- cterized by high sensitivity and resolution and more accurate d-values, thus greatly promoting the "indexability" of a Guinier pattern. By comparing the techniques of Debye-Scherer camera and diffractometer, the advantages of the monochromator technique is emphasized, and some experimental methods for obtaining d spacings of high precision are proposed.