A transmission/reception method and apparatus in an Orthogonal Frequency Division Multiplexing (OFDM)-based wireless communication system are provided, in which a data transmitter adaptively selects a multiplexing scheme according to a code rate of the transmission packet or an effective SNR of a wireless channel for a corresponding terminal, when transmitting a packet data channel. The adaptive multiplexing scheme is equal in concept to a scheme of adaptively implementing unitary precoding. The unitary precoded multiplexing includes Orthogonal Frequency code Domain Multiplexing (OFCDM), Fast Fourier Transform-Spread-OFDM (FFT-S-OFDM), Fast Frequency Hopping-OFDM (FFH-OFDM), and the like. The method and apparatus adaptively selects the multiplexing scheme and transmits/receives data using the selected multiplexing scheme, thereby improving reception performance of the packet data.