We study the roles of graphene acting as a buffer layer for growth of an A1N film on a sapphire substrate. Graphene can reduce the density of A1N nuclei but increase the growth rate for an individual nucleus at the initial growth stage. This can lead to the reduction of threading dislocations evolved at the coalescence boundaries. The graphene interlayer also weakens the interaction between AIN and sapphire and accommodates their large mismatch in the lattice and thermal expansion coefficients; thus, the compressive strain in A1N and the tensile strain in sapphire are largely relaxed. The effective relaxation of strain further leads to a low density of defects in the AIN films. These findings reveal the roles of graphene in III-nitride growth and offer valuable insights into the efficient applications of graphene in the light-emitting diode industry.