An oil pressure control system of an automatic transmission for a vehicle includes a lock-up clutch, a TC regulator valve, an LC control valve, an LC shift valve, an oil cooler, a control area determination device, a torque transmission capacity calculator, a high load state determination device, and an oil pressure controller. The oil pressure controller is configured to control a TC regulator pressure outputted by a TC regulator valve, so that a torque transmission capacity of a lock-up clutch is greater than a reference torque transmission capacity calculated by the torque transmission capacity calculator, if a control area determined by the control area determination device is a complete engagement area and the high load state determination device determines that an operational state is in a high load state.