A liquid low level warning device is used on the nut of a screw bolt or the slide block of a linear guideway, and comprises: a circuit terminal, rail and buoyage. The terminal is disposed on a lubricating device in such a manner that an end of the terminal is electrically connected to external equipment (by wire or wireless means) and another end is connected to the rail and positioned in the receiving space of the lubricating device. A stop block is provided on the upper end of the rail and the buoyage is moveably mounted on the rail. With such arrangements, the liquid low level warning device will produce alarm signals when the lubricating oil is lowered to a predetermined level.