A fastening assembly is for securing a heat sink to a heat generating electronic device located on a printed circuit board. The fastening assembly includes a first element and a second element slidably received in the first element. The first element includes a pressing portion, a plurality of outer flexible portions located below the pressing portion and a plurality of inner flexible portions integrally extending inwards from the outer flexible portions, respectively. The pressing portion of the first element presses downwardly the heat sink toward the electronic device. The outer flexible portions of the first element abut upwardly against the printed circuit board. The second element is capable of acting on the inner flexible portions of the first element to motivate the outer flexible portions to release their abutment from the printed circuit board.