A heat dissipation device includes a heat sink having a first fin unit and a second fin unit adjoining each other. The first fin unit includes a plurality of parallel stacked first fins each including a main body and two flanges extending perpendicularly from two opposite sides of the main body. The second fin unit includes a plurality of parallel stacked second fins each including a main body and two flanges extending perpendicularly from two opposite sides of the main body. The first fin unit and the second fin unit are arranged back-to-back. The main body of an outmost first fin adjacent to the second fin unit abuts the main body of an outmost second fin adjacent to the first fin unit. An interval between two neighboring first fins is smaller than that between two neighboring second fins.