An LED assembly includes a substrate and a plurality of LEDs mounted on the substrate. Each LED comprises an LED die mounted on the substrate via an adhesive, a base spacedly surrounding the LED die, a pair of leads inserted in the base to be in electrical connection with the LED die, and an encapsulant sealing the LED die and inner parts of the leads therein. A thickness of the adhesive is selected to be less than 0.01 inches. The substrate contains a kind of coolant therein to rapidly remove heat from the LED die to atmosphere.