A hydraulic pressure supply system of automatic transmission may include a low-pressure hydraulic pump receiving the oil stored in the oil pan and generating the low hydraulic pressure, a low-pressure regulator valve controlling the low hydraulic pressure of the first low-pressure line, a reducing valve fluid-connected to the low-pressure regulator valve and supplying a hydraulic pressure to the low pressure portion, a high-pressure hydraulic pump increasing the low hydraulic pressure supplied from the low-pressure hydraulic pump, a switch valve disposed between the first low-pressure line and the high-pressure line, and selectively connecting the first low-pressure line and the high-pressure line, a high-pressure regulator valve controlling the increased hydraulic pressure supplied from the high-pressure hydraulic pump and the low hydraulic pressure selectively supplied from the low-pressure hydraulic pump through the switch valve, and supplying the stable high hydraulic pressure to the high pressure portion.