A tool capable of turning on/off a light automatically comprises a body including a fitting area in response to a fitted tool and a receiving groove disposed therein to communicate with a font end thereof; a light module installed in the receiving groove and including at least one bulb and a cell set, a first electrode of the cell set connecting with a first connecting leg of the bulb, and a second electrode of the cell set connecting contacting with the fitting area of the body, and when the fitting area of the body is fitted to a fitted tool, a second connecting leg of the bulb conducting to the second electrode of the cell set.