A power transmission system for a four-wheel drive vehicle comprises a power transmission mechanism provided in a vehicle arranged such that power is always transmitted to one of two pairs of wheels consisting of a pair of front wheels and a pair of rear wheels, to transmit power to the other pair of wheels, and a differential gear unit arranged between left and right wheels in the other pair, to distribute the power transmitted from the power transmission mechanism to the other pair, between the left and right wheels. The differential gear unit is a torque-sensitive differential gear unit with a differential limiting function for producing a differential limiting force between the left and right wheels, depending on power transmitted thereto, and a power transmission control means controls the power transmitted from the power transmission mechanism to the differential gear unit according to a driving state of the vehicle.