A control system for a stepless variable speed transmission of power splitting type for use in a vehicle in which the power from an engine is transmitted with a controllable speed and wherein part of the power is converted into hydraulic power by means of hydraulic pump motor. The system comprises an up shift valve which switches a hydraulic pressure of a clutch and brake in the transmission to change the transmission to a high speed ratio range when a speed ratio increasing hydraulic pressure in a speed ratio control actuator exceeds a given value, a down shift valve which switches the hydraulic pressure of the clutch and brake to change the transmission to a low speed ratio range when a speed ratio decreasing hydraulic pressure exceeds a given value and a directional control valve which reverse the direction of the speed ratio increasing and the speed ratio decreasing hydraulic pressure to the actuator in accordance with a pressure difference between the hydraulic pressure of the clutch and brake in the low speed ratio range and the hydraulic pressure of the clutch and brake in the high speed range.