A heat dissipating device includes a fastening structure and a heat sink. The heat sink has a thermal conductive base which has a top surface and two T-shape slots formed at two opposing ends of the top surface. The fastening structure has a pair of brackets. Each of the brackets has a planar plate and a T-shape arm to be inserted into the corresponding T-shape slot. The T-shape arms are hinged with one end of the planar plates, such that when the T-shape arms are inserted into the T-shape slots, the planar plates extend horizontally at a level lower than the top surface of the base. Each of the planar plates has a through hole through which the fastening structure can be mounted to a board by a fastener.