A system for controlling distribution of drive forces from a drive source unit to front and rear wheels of a four-wheel drive motor vehicle includes drive force distributing means for transmitting the drive forces to the front and rear wheels at a continuously variable distribution ratio, first speed detecting means for detecting a rotational speed (Nf) of the front wheels, second speed detecting means for detecting a rotational speed (Nr) of the rear wheels, and control means for calculating a slip ratio (S) of the rear wheels to a road on which the motor vehicle is running based on the rotational speeds (Nf, Nr) of the front and rear wheels, calculating a distribution ratio between the drive forces to be distributed to the front and rear wheels based on the slip ratio (S), and controlling the drive force distributing means to transmit the drive forces from the drive source unit to the front and rear wheels according to the distribution ratio. According to a method of controlling distribution of drive forces to the front and rear wheels, a continuously variable optimum distribution ratio is determined in view of a slip ratio between the wheels and the road, and the drive forces from the drive source unit are distributed and transmitted to the front and rear wheels according to the distribution ratio.