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The fabrication of GaN-on-diamond structure by the process of diamond growth on GaN technology is becoming more and more attractive for high-power GaN devices. However, due to the high-temperature steps employed during the GaN epitaxial layer transfer process, significant stress accumulates in the GaN-on-diamond structure and induces the formation of cracks or deformations in the GaN layer. To improve the stress characteristics of the GaN layer, in this work, dual-sided diamond film deposition was carried out to accomplish the GaN-on-diamond structure. The surface morphology, interface microstructure and crystal quality of diamond on GaN were analyzed by using scanning electron microscopy, atomic force microscopy, transmission electron microscopy and X-ray diffraction. The results showed that the stress accumulation originating from the GaN epitaxial layer transfer process was effectively alleviated as compared with other reporteds in the literature and that the crystal quality of the GaN was only slightly altered. The stress relaxation was attributed to the low-quality diamond used as the temporary carrier during the GaN transfer process. In addition, a thin SiC transition layer in the interface is also beneficial in alleviating the mismatch between the coefficients of thermal expansion for the GaN and the diamond.