Disclosed is an all-terrain vehicle (ATV) engine power output conversion mechanism, which is characterized primarily by linking a first transmission assembly to the output terminal of the engine, where the central shaft of the first transmission assembly is provided with a first gear at its one end which connects to the output terminal of the engine, while the other end of the central shaft to a first bevel gear; having a second transmission assembly, where its central shaft is provided with a second bevel gear at its one end which gears to the first bevel gear, while the other end of the central shaft to a second gear, and the central shaft is joined to the rear wheel axle to drive the rear wheels; having a third transmission assembly, where its central shaft is provided with a third gear which gears to the second gear, and the third gear gears to a reduction gear, where the output terminal of the reduction gear is used to drive the front wheels; and having a switch set up at the handle bar of the ATV or wherever the rider can reach, where the switch is for the control of the reduction gear to drive the front wheel. Through such setup, the ATVs are capable of selecting a two-wheel drive or a four-wheel drive, subject to the demand upon various terrain conditions.