A heat sink fastening assembly includes a fastener comprising a latching member and an operating member. The latching member includes a pressing part, a first latching leg and a second latching leg bent downwards respectively from two opposite ends of the pressing part. The operating member has a coping and a sidewall extending downwards from an edge of the coping. The sidewall defines two locking slots engaging with the first latching leg and an elastic rib extending downwards from the coping and engaged with the first latching leg of the latching member for inhibiting any movement of the operating member in a direction along the locking slot of the sidewall.