An electronic system such as a computer system includes a casing defining an opening at a side thereof, a motherboard arranged in the casing, a hard disk located at a side of the motherboard and a heat dissipation structure covering the opening of the casing. The motherboard includes a printed circuit board facing toward the opening of the casing, first electronic components and second electronic components mounted on the printed circuit board and facing toward the opening. The heat dissipation structure includes a base engaging with the casing and fins extending from the base and outside of the casing. The base includes a first engaging portion contacting the first and second electronic components and a second engaging portion contacting the hard disk. The first engaging portion and the second engaging portion are in different levels from each other.