Epitaxial film quality is critical to the success of high-performance a-Ga 2 O 3 vertical power devices. In this work, the origins of threading dislocation generation and annihilation in thick a-Ga 2 O 3 films heteroepitaxially grown on sapphire by the mist-CVD technique have been examined by means of high-resolution X-ray diffraction and transmission electron microscopies. By increasing the nominal thickness, screw dislocations exhibit an independent characteristic with a low density of about 1.8 Â 10 6 cm À2 , while edge dislocations propagating along the c-axis are dominant, which decrease down to 2.1 Â 10 9 cm À2 in density for an 8 lm-thick a-Ga 2 O 3 layer and exhibit an inverse dependence on the thickness. In the framework of the glide analytical model, parallel edge dislocations are generated at the interface due to the misfitinduced strain relaxation, while the dislocation glide and coalescence result in the annihilation and fusion behaviors. The optimal thick a-Ga 2 O 3 with low dislocation densities may provide a prospective alternative to fully realize a-Ga 2 O 3 power devices.