A heat dissipation device for being installed to and cooling an electronic card device (40) includes first and second heat dissipation units (10, 20) and at least one heat pipe (30). The heat dissipation units each include a base plate (12, 22) and a fin plate (14, 24) engaged with the base plate. The at least one heat pipe includes an evaporating segment (32) received between the base plate and the fin plate of the first heat dissipation unit, and a condensing segment (34) received between the base plate and the fin plate of the second heat dissipation unit, thereby connecting the first and second heat dissipation units together. The evaporating segment and the condensing segment of the at least one heat pipe are movably received in the first and second heat dissipation units respectively before the heat dissipation device is installed to the electronic card device and dissipating heat generated therefrom, and are firmly secured in the first and second heat dissipation units respectively after the heat dissipation device is installed to the electronic card device and dissipate heat generated therefrom.