A system for controlling a power transmission of a four-wheel drive vehicle powered by an engine. The system is provided with a transmission for transmitting a power of the engine to main wheels, a clutch for selectively transmitting the power to auxiliary wheels, a vehicle speed sensor for sensing the speed of the vehicle and for producing a vehicle speed signal dependent on the speed, a steering angle sensor for sensing the steering angle of the vehicle and for producing a steering angle signal dependent on the steering angle. A control circuit is provided to compare the vehicle speed signal and steering angle signal with reference values and judge whether the condition is in the range of conditions causing tight corner braking and to produce a clutch signal. A switch is responsive to the clutch signal for disengaging the clutch, whereby the four-wheel drive is changed to the two-wheel drive.