Disclosed is a hydraulic control system of an automatic transmission used in a vehicle, which includes an oil pump, a manual valve for selecting one of the parts of a hydraulic pressure line to deliver the hydraulic pressure produced by the oil pump according to the vehicle speed range, a first to a fourth pressure control valve with a first port to receive the pressure supplied by the manual valve in a forward drive range, a first to a fourth solenoid valve for respectively controlling the pressure control valves according to a vehicle speed, a rear clutch valve for receiving a hydraulic pressure from the pressure control valves, a low/reverse valve, a fail-safe valve, and a high-low pressure valve for receiving a hydraulic pressure from the fourth pressure control valve to vary the line pressure.