An apparatus and a method for expanding an operation region in an envelope tracking power amplifier are provided. The apparatus for amplifying power of a transmission signal includes an amplitude component determination unit, a supply modulator, and a power amplify module. The amplitude component determination unit determines an amplitude component of a transmission signal. The supply modulator generates a supply voltage to be provided to the power amplify module depending on the amplitude component of the transmission signal determined by the amplitude component determination unit. The power amplify module amplifies power of the transmission signal depending on the supply voltage generated by the supply modulator.