A fastening device for a heat sink includes a one-piece shaft. The shaft includes a head at one end thereof, an engaging section at an opposite end thereof, a connecting section between the head and the engaging section, and a tray extending above the engaging section. A spring surrounds the connecting section between the head and the tray of the shaft. A gasket engages with the connecting section of the shaft and is retained between the spring and the tray of the shaft. In this way the shaft, the spring and the gasket are assembled. The gasket extends a downward, annular sidewall for engaging in the heat sink, by which the fastening device can be stably fixed to the heat sink. A back plate extends a nut toward the shaft for engaging with the engaging section of the shaft.