A mortise lock is provided with a reinforcing plate to enhance structural rigidity and robustness for resistance against external destructive pulling forces. The reinforcing plate is disposed in abutment with an exterior cover and a door panel, and is mounted on a mounting post of the exterior cover. The reinforcing plate has a notch to receive and engage a key lock. A securing plate may be disposed between and in abutment with the door panel and a main body of a torsional returning unit which is connected to an exterior handle to further increase structural rigidity.