Abstract: A modular door lock apparatus having a mainframe defined by a front face plate and an outer and inner side plate extending rearwardly of said front face plate a latch assembly being fixedly mounted to said front face plate, so as to be positioned between the side plates. A key cylinder is secured to the outer side plate and a latch knob is mounted to the inner side plate so as to operate the latch bolt of the latch assembly. The door lock apparatus is secured to the door by sliding the apparatus into a central bore and adjacent longitudinal slots that are formed in the leading edge of the door. The apparatus is then secured to the door by means of a pair of transversely positioned screws that are received through the oppositely disposed slots having their threaded ends extending outwardly of the outer side plate to engage the key cylinder. The screws may also be provided with nuts, which are fixedly positioned against the outer side plate.