An adjustment mechanism is used in an optical recording and/or reproducing apparatus for fixing the position of a tray. A slider moved along left-right direction in the optical recording and/or reproducing apparatus is corresponding the tray moved along front-back direction. The adjustment mechanism comprises a L-shaped chute located on the tray, and comprises a guide member and an arm located on the slider. When the tray get into the optical recording and/or reproducing apparatus, the guide member slides into the L-shaped chute. Then the arm contacts the outside surface of the L-shaped chute. Whereby, the tray be fixed on a suitable position.