A positioning mechanism for a quick release device of a slide assembly includes an opening in the rail and a positioning member located corresponding to the opening. The rail has a first contact portion adjacent to the opening which has an open portion and a reception portion communicating with the open portion. The positioning member includes a base, a resilient arm, a restriction portion and a second contact portion. The base is fixed to the rail. The resilient arm is connected between the base and the restriction portion. The restriction portion has a first contact end located corresponding to the open portion. The second contact portion is connected to the restriction portion. An substantially enclosed area is defined between the first contact end and the reception portion. The second contact portion contacts the first contact portion of the rail such that the first contact end is secured.