A fixing frame for a heat sink includes boards, at least one foldable plate, and connection members. The boards surround a hollow area, and the hollow area includes a main accommodating space and at least one subsidiary accommodating space at one side of the main accommodating space. At least one foldable plate is detachably connected to one of the boards and is disposed in the subsidiary accommodating space. The connection members are disposed on the boards. By means of the connection members, the fixing frame mounts the heat sink on a circuit board of various sizes to dissipate heat therefrom.