A hydraulic pressure supply system of an automatic transmission may include a mechanical oil pump driven by an engine to supply the oil stored in the oil pan to a first supply path fluidly-connected to the mechanical oil pump, a switch valve fluidly-connected to the first supply path and selectively supplying oil supplied from the first supply path to the transmission unit through a second supply path fluidly-connected to the switch valve and the transmission unit, a solenoid valve actuator engaged to the switch valve, and an electrical oil pump fluidly connected to the oil pan to pump the oil stored in the oil pan as a second high-pressure hydraulic pressure and supply the oil in the pan to a third supply path fluidly-connected with the second supply path, wherein the first and second hydraulic pressures may be applied to the transmission unit through three operation modes.