A heat dissipation device includes a first heat dissipation unit attached to a top surface of the electronic component of an add-on card and defining a channel therein, a heat pipe, and a second heat dissipation unit. The second heat dissipation unit includes a first portion located at a lateral side of the add-on card, a second portion extending from the first portion to a bottom side of the add-on card, and a joint portion formed between the first portion and the second portion and located at the lateral side of the add-on card. The heat pipe includes an evaporating portion received in the channel of the first heat dissipation unit, and a condensing portion extended through the second heat dissipation unit at the joint portion.