With an aim to understand the great reduction in the X-ray diffraction (XRD) intensities of high-entropy alloys, a series of Cu-Ni-Al-Co-Cr-Fc-Si alloys with systematic addition of principal elements from pure element to seven elements was investigated for quantitative analysis of XRD intensities. The variation of XRD peak intensities of the alloy system is similar to that caused by thermal effect, but the intensities further drop beyond the thermal effect with increasing number of incorporated principal elements. An intrinsic lattice distortion effect caused by the addition of multi-principal elements with different atomic sizes is expected for the anomalous decrease in XRD intensities. The mathematical factor of this distortion effect for the modification of XRD structure factor is formulated analogue to that of thermal effect. (C) 2007 Elsevier B.V. All rights reserved.