Abstract: The misorientation between epitaxial high-Tc superconducting film of Yba2 CuaO_7-x （YBCO） and the substrate of SrTiO₃ （STO） and the crystalline perfection of the film have been studied by X-ray double-crystal diffraction. The misorientation between the film and substrate along the c-axis is not due to the lattice mismatch. It changes with ϕ, the angle of rotation around the normal to the film surface, and has a maximum on one of ＜110＞ directions of the substrate. The FWHMs （full widths at half maximum） of the diffracted peaks in the rocking curves of the film,indicate that the epitaxial film is not so perfect and the FWHMs change with the rotation angle ϕ and has maximum or minimum along ＜110＞. The above facts are probably due to the preferred orientation of twin boundaries and the misorientation between mosaics.