A lid mechanism for an interface slot opening of notebook computers includes a notebook computer body, which has an interface slot opening on a lateral side and a first sliding trough formed on one side of the opening, and a lid located on the interface slot opening in a turning manner to close the opening. The lid includes a lid body, which has a recess on the surface in the center, an inner surface attaching to a water-proof plate and a second sliding trough on an upper side corresponding to the first sliding trough. The recess has a lever fastening to a latch arm and driving the latch arm, to move in the second sliding trough. When the lid is in a closed condition to compress the waterproof plate, the latch arm is wedged gradually in the first sliding trough.