A transmitting/receiving apparatus and method in a mobile communication system are provided. In a transmitter, an encoder encodes input information data and outputs systematic bits on a first channel and redundancy bits on a second channel. A first channel interleaver interleaves the systematic bits and a second channel interleaver interleaves the redundancy bits. A modulator maps one channel data of I and Q channel data, received from the first channel interleaver and another channel data of the I and Q channel data, received from the second channel interleaver to a modulation symbol using different mapping rules for I and Q channels.