Abstract: In this paper, the various factors affecting the reselving power of diffraction lines on the equator of monochromatic X-ray focussing photograph are analyzed in detail. The relationship equations and calculated results of the broadening of diffraction lines caused by wavelength dispersion of X-ray characteristic spectra, wavelength difference of Kα-doublet, quality and inexact focussing of monoohro-mator, specimen thickness and grain size are given. According to the calculated and experiment results, the synthetic factor affecting the resolving power of diffraction line is discussed.