Abstract: The present invention discloses an improved structure for a mortise lock comprising a lock body, a lock core, a plurality of spring washers, a plurality of straps and a latch in the lock body. Each spring washer has a curved portion to hook onto the washer holding slot, a claw to hook onto the strap tenon or the washer holding slot and a rim slot to forcibly contact the positioning pin and indirectly hold the strap tenon. With such mechanism a resilient force may be produced to prevent the straps from being shifted out of their positions due to vibration even after the lock is opened. This improved structure makes the mortise lock of the present invention can not only be used on static objects such as telephone sets or mail boxes, but also effectively applicable to any moving object.