The past results related to the main features of formation of structural V-defects in polar GaN(0001) and semi-polar epitaxial GaN films are presented. The GaN films have been synthesized by hydride-chloride vapor-phase epitaxy (HVPE) on two different substrates Si(111) and Si(100) with an intermediate nano-SiC epitaxial layer. The nano-SiC layer has been formed by a method of atom substitution on the Si substrate. The experimental studies have demonstrated that V-defects on the surface of GaN(0001) films are regular hexagons of 30 µm in size, while on the surface of GaN V-defects are of the order of 1 µm. It was found that V-defects on the semipolar face are extended along the (1123). direction. The size of oblique facets of V-defects on the surface of polar GaN(0001) is of about 1 µm, while on the surface of semipolar GaN (1101) layer they are much less, measuring about 150 nm, on the average. On the basis of thermodynamics, the mechanisms of nucleation of V-defects on polar and semipolar faces of GaN epitaxial films are elucidated, and the criteria of the formation of V-defects are theoretically derived. A good qualitative agreement between the experimental results and the theoretical model was found.