A lock and alarm device includes: a lock body held on a window and door rail at a lower position of the door or window for an easier concealment, a vise having two jaw members telescopically mounted in a bottom of the lock body to be firmly tensioned or retained on the rail as driven by a knob for retarding a sliding opening of the window and door, and an alarm mounted on the lock body and operatively actuated for sensitively detecting an unexpected opening of the window and door for enhancing security when the opening of the window or door triggers a touch-on switch of an alarm circuit, or when the vibrations caused by the opening of the window or door actuates a vibration sensor of the alarm circuit.