A semiconductor device including a substrate, a semiconductor package, a plurality of pillars and a lid is provided. The semiconductor package is disposed on the substrate and includes at least one semiconductor die. The plurality of pillars are disposed on the semiconductor package. The lid is disposed on the substrate and covers the semiconductor package and the plurality of pillars. The lid includes an inflow channel and an outflow channel to allow a coolant to flow into and out of a space between the substrate, the semiconductor package, the plurality of pillars and the lid. An inner surface of the lid, which faces and overlaps the plurality of pillars along a stacking direction of the semiconductor package and the lid, is a flat surface.