Disclosed are an apparatus and method for transmitting a signal in a wireless communication system. The apparatus includes a controller for receiving power control information of a baseband signal, deciding an output mode, and providing an output mode signal, a signal converter for receiving the baseband signal outputting a phase signal, and outputting an envelope signal when the output mode signal indicates a first output mode, a phase modulator for up-converting the phase signal, and an amplifier for combining the envelop signal and the up-converted phase signal for the first output mode and amplifying the combined signal.