Abstract: The accurate measurement of peak position, peak shape and peak intensity in Bragg-Brentano geometric powder diffraction determines the quality of the XRD pattern, and is also the premise and key to correctly characterize the crystal structure of the material. The intensity of the diffraction peak will be directly affected by the size of the divergence slit in the primary beam path, and the peak shape of diffraction pattern of the sample with small size will be significantly affected by the unreasonable setting of the divergence slit. The effect of the width and length of the divergence slit on the peak-to-background (P/B) ratio in diffraction patterns of the sample with small size was studied. The results show that the width and length of the beam spot on the sample were determined by the width and length of the divergence slit. The backgrounds of diffraction patterns increased remarkably at lower or higher angle regions with the increasing width and length of the beam spot on the sample. That is to say, the P/B ratio reduced significantly with the increasing width and length of the divergence slit. Therefore, the width and length of the divergence slit were appropriate to be small than the actual size of the sample with small size due to the horizontal and vertical divergence of the spot.