Abstract: The crystal perfection of thick Hg_1-xCd_xTe layers grown by the vertical dipping LPE on CdZnTe substrates has been studied by the X-ray double crystal rocking curve （DCRC） analysis. The structural quality of Hg_1-xCd_xTe top epilayers is better than that of the region near interface between the substrate and epilayer. The full width at half maximum （FWHM） of DCRCs was measured on a cross array of points over epilayer surface. The dislocation density of Hg_1-xCd_xTe thick epilayer is calculated using the FWHM values, therefore the map of the structural uniformity of Hg_1-xCd_xTe epilayers can be obtained in this way. The structural uniformity of the thick Hg_1-xCd_xTe epilayers is also studied in the growth direction.