A method and apparatus is provided for transmitting feedback information by each of user terminals in a communication system including the user terminals with different numbers of antennas. A multicast data packet is received from a base station through a downlink multicast transmission channel. If the multicast data packet has never been successfully decoded before, the multicast data packet is decoded and the success/failure in decoding the multicast data packet is determined. Upon the failure in decoding the multicast data packet, a NACK signal indicating the failure in decoding the multicast data packet is generated with sequences, the number of which is equal to the number of antennas of the user terminal. The NACK signal is transmitted to the base station through a shared feedback channel. If the user terminal has at least two antennas, sequences corresponding to the at least two antennas are orthogonal to each other and constitute sequence signals.