A power transmission system for a four-wheel drive vehicle has front and rear wheels driving power transmission sub-systems for transmitting the output of engine (1) to front and rear wheels (23). A forward overrunning clutch (30) and a reverse overrunning clutch (31) are provided in one of the power transmission systems. One of the overrunning clutches can be selected in accordance with the operation of a driver of the vehicle. The overrunning clutch operates to transmit the power to corresponding wheels and to allow the selected wheels to rotate faster than the other wheels thereby preventing tight corner braking. A locking device (34) is provided for locking both the overrunning clutches.