Abstract: A positioning device for a sliding mechanism that includes exterior, intermediate and interior plates, each plate defining inner and outer side surfaces, the positioning device is characterized by: the intermediate plate having a stop block formed on the inner side surface; and the interior plate including an elongated release rod that is mounted movably on the outer side surface and that has a limit notch receiving the block, and a resilient element engaging the propeller portion for restricting a longitudinal movement of the rod, the interior plate having a position limiting unit constituted by two limiting stubs mounted movably on the outer side surface and having pointed free ends and a biasing element for biasing the limiting stubs in such a manner that the pointed free ends extend into the notch to contact opposite ends of the block, thereby preventing relative movement between the interior and intermediate plates.