A deadlock includes a casing, a front knob and a rear knob respectively rotatably formed on the front and rear walls of the casing, and a slidable latch reciprocatively mounted in the casing as driven by either the front or rear knob, wherein the latch is formed a triangular locking tongue having two inclined surfaces so that the latch can be conveniently used in a bi-directional door when opening the door either outwards or inwards.