An LED illuminating device includes a mounting module and a lamp unit mounted in the mounting module. The lamp unit includes a light-emitting module and a heat sink. The light-emitting module includes a light source having a plurality of LEDs, and a light penetrable cover located below the light source and defining a plurality of air venting holes therein. The heat sink includes an elongated base defining a plurality of air exchanging holes therein and a plurality of fins. The base has an outer convex surface and an opposite inner concave surface defining an elongated recess. The light source is received in the recess and thermally attached to the concave surface. Air flows into and out of a chamber defined between the base and the cover via the venting holes of the cover and the air exchanging holes of the base.