We report direct in situ selective growth of hierarchical GaN block-like nanoflakes on the graphene nanosheets without a seed/catalyst. The selective growth behavior is attributed to surface dangling bonds and edge terraces of graphene nanosheets, especially the edge terraces on the boundaries can play a important role on the position constrain of grown GaN nanoflakes. The GaN showed polycrystalline hexagonal wurtzite structure, and with the morphology, diameter and density depended strongly on the gas flow rate. The suitable gas flow rate results in the appropriate nucleation of GaN on the rGO nanosheet and the adatoms migrated to their suitable position towards the [11-22] direction, thereby producing in situ block-like GaN nanoflakes selective growth without assist of a seed or catalyst. Furthermore, the GaN nanoflakes/rGO hybrids suggests good UV-vis absorption and photoluminescence properties. These results demonstrate it provides a efficient route for the growth of hierarchical GaN nanostructures on the graphene nanosheets, which are highly desirable for fabricating GaN/rGO hybrid optoelectronic devices, particularly for photodetectors, sensors and photocatalysis.