An LED lamp assembly includes a bracket, a heat sink secured on the bracket, a plurality of LED modules mounted on a bottom of the heat sink, and a transparent envelope correspondingly covering the LED modules. A sealing chamber is cooperatively defined by the heat sink, the bracket and the envelope, and the LED modules are received in the sealing chamber. The LED assembly is further provided with a gas-injecting member for injecting a predetermined gas into the sealing chamber of the LED lamp assembly therethrough.