A retaining device for a heat sink includes a frame, at least an operation member and at least an engaging member. The frame is attached to an upper portion of the heat sink with a projection at two opposite sides thereof and the projection has a contact face. The operation member provides a main operation part and a stir part. The main operation part is disposed on top of the contact face and the stir part is actuated to rotate for the main operation part being capable of moving relative to the contact face. The engaging member further has a follower part piercing the projection with an end of the follower part connecting with the main operation part pivotally and another end of the follower part being joined to a first elastic part and a second elastic part respectively. The first elastic part and the second elastic part oppositely extend horizontally and free ends of both the elastic parts are joined to two opposite lateral sides of an engaging part for controlling the engaging force evenly and decreasing plastic deformation.