Abstract: HgCdTe/CdZnTe heterostructures with -0.044%lattice mismatch grown by Liquid Phase Epitaxy were studied by reciprocal space maps of high resolution X-ray diffraction. The results showthat HgCdTe epilayer with more than 10μm thickness is fully relaxed and the orientation of epilayer has 0.01o tilt compared with that of the substrate. However, due to the composition gradient of HgCdTe epi-layer and the difference of the coefficients of thermal expansion between HgCdTe epilayer and CdZnTesubstrate, stress and strain still exist in HgCdTe epilayer near the interface. In addition, it is observed that in the reciprocal space maps both for the symmetric and asymmetric diffractions the maps of HgCdTe epilayer have a broadening perpendicular to scattering vectors of reciprocal space maps. It means thatthe mosaic structure of HgCdTe epilayer is formed during the relaxation process. On the other hand, thedeviation of the broadening direction of reciprocal space maps in asymmetric diffraction from scattering vectors is found and it can be explained by the gradients of composition and strain in HgCdTe epilayer based on the relaxation line model.