An LED lamp comprises a heat sink with a plurality of LED modules attached thereon, an envelope receiving the heat sink and the LED modules, and a top cover and a bottom cover mounted at two ends of the heat sink via a plurality of fasteners and shafts with an elastic member coiled therearound. The top cover, the envelope and the bottom cover cooperatively form a sealed space receiving the heat sink and the LED modules. When the fasteners are fastened toward the heat sink, the shafts move upwardly relative to the bottom cover, whereby a distance between the top cover and the heat sink is decreased, so the heat sink makes tight contact with the top cover.