A mounting device for a heat sink (200) includes a pair of pivot members (10), two joining members (30), and two pairs of spring members (20). Each pivot member includes a clipping portion (11) for clasping the electric unit (300). The joining members pivotably connect with the pivot members. Each joining member includes a pressing portion (310) for resiliently abutting against the heat sink toward the electric unit. The spring members each includes two end catches (21), (22) secured with the joining member and a corresponding member and capable of providing resilient torsions to drive the clipping portions to engage with the electric unit when the clipping portions rotate to one position away from the electric unit.