Morphology as well as the dislocation networks in epitaxial GaN thin film, prepared via selectively lateral overgrowth has been characterized using TEM combined with focused ion beam (FIB) tool. The results showed that orientations of the sidewalls dependent on the orientations of mask strips. The sidewalls coincide with the {11 (2) over bar1} planes that form V type voids when the mask strips aligning along [1 (1) over bar 01] directions and correspond to the {1 (1) over bar 00} planes that result in rectangular voids if the strips arranging along the [11 (2) over bar0] directions. The dislocations were observed along the plan view direction. The dislocations in the lateral overgrown region mainly developed along the direction perpendicular to the strips. The genetic aspect of such morphologies of GaN films may have very close relation with the change of growing fronts of the epitaxial layer.