An method and apparatus for power amplification in a mobile communication system, wherein a signal converter for receives signals from a baseband-side, modulates the received signals into oversampled baseband digital signals, and generates signals having a constant envelope with the oversampled base band digital signals, an up-converter up-converts the generated signals having the constant envelope to Radio Frequency (RF) signals for output, and a power amplifier amplifies the RF signals according to system setup levels and outputs envelope signals of a predetermined level.