An electronic ballast is provided with an elongated housing; a heat sink module disposed in the housing and including a channel extending from one end to an other end; a plurality of power semiconductor modules disposed on both sides of the heat sink module respectively; an inlet assembly disposed at one end of the heat sink module proximate to a first panel at one end of the housing; an outlet assembly disposed at the other end of the heat sink module proximate to a second panel at an other end of the housing; and a fan disposed between the other end of the heat sink module and the second panel. Only a small portion of heat is dissipated by surface areas of the housing. Thus, the housing can be made compact to save space.