This paper deals with a center-closed rotary servo valve in a hydraulic power steering (PS) system having an improved straight line stability during driving.Conventional rotary servo valves are of a center-open type and are poor in straight line stability because the assist pressure to steer occurs with even slight steering inputs.In the vicinity of neutral position the center-closed rotary servo valve does not produce assist pressure and leaves room for manual steering effort. The center-closed rotary servo valve is composed of a center-closed circuit and a center-open circuit in parallel. The center-closed circuit precisely controls the direction of oil flow to the cylinder chamber A or B in the vicinity of neutral position. The chamfers formed at center-closed metering edges control a lower level of assist pressure. Also the change of assist pressure from manual- to power-steering is smooth.So we have investigated characteristics of a steering gearbox with a center-closed rotary valve both on a test bench and on an actual vehicle. Test results demonstrated large improvements in straight line stability as well as steering feeling.