Provided are an apparatus and method for processing an ARQ in a MIMO system. The apparatus includes a storing unit for storing reordering thresholds of each receiving terminal and retransmission data; a priority managing unit for assigning a priority to a plurality of antenna groups in ascending order of a bit error rate of a wireless channel of each antenna group and managing the plurality of antenna groups based on the priorities; a data transmitting unit for transmitting retransmission data; and a controlling unit for controlling the data transmitting unit to transmit retransmission data within a range not exceeding a reordering threshold of a corresponding receiving terminal sequentially using an antenna group having the highest priority first.