With area detector X-ray diffraction system, the orientation of superconductor films and their relationships with substrates can be easily examined. For YBCO high temperature superconductor films having complex crystalline orientation relationship fore and after melting process, 2-dimesional X-ray diffraction provides us much more information than common X-ray diffraction. This is very helpful for understanding the peritectic reaction during melting process because the growth mechanism of Y211 crystalline can be explained from its orientation information. Y211 phrase in the film and its orientation relationship with YBCO phase and MgO substrate have been analyzed at first. It is found that Y211 crystalline is well match with both MgO substrate and YBCO parent phase. Then the melting process of YBCO has been discussed from the view of the orientation relationship between each phase.