A heat dissipation device assembly includes a heat sink (20) placed on an electronic component (52) which is mounted on a printed circuit board (PCB) (50) and including a pair of shoulders (23) on opposite sides thereof, a pressing part (30) including a pair of pressing portions (36) supported on the shoulders and a pair of locking portions (34) spaced from the shoulders, a sliding part (40) slidably attached to the pressing part and including a pair of locking portions (41) spaced from the shoulder, and a back plate (60) mounted below the PCB and including four posts (61) extending through the PCB and the heat sink to resilently engage with the locking portions. Each locking portion defines a locking opening (33, 43) including an entrance (33a, 43a) and a locking slot (33b, 43b), the post being capable of extending through the entrance to engage in the locking slot.