An LED lamp includes a hollow, prism-shaped heat sink (30), a plurality of LED modules (20) attached on sidewalls (320) of the heat sink, a cover (40) mounted on a top of the heat sink, a lampshade (10) secured to a bottom of the heat sink, and a fan (50) fixed on the top of the heat sink and through the cover. A plurality of fins (340) extends inwardly from inner faces of the heat sink to cooperatively define two crossed slots in the heat sink. When the fan is in operation, an airflow produced by the fan flows through the fins from the top through the bottom of the heat sink, thereby dissipating heat absorbed by the heat sink from the LED modules rapidly and efficiently.