A digital television (DTV) transmitter/receiver and a method of processing data in the DTV transmitter/receiver are disclosed. In the DTV transmitter, a pre-processor pre-processes the enhanced, data by coding the enhanced data for forward error correction (FEC) and expanding the FEC-coded enhanced data. A data formatter generates enhanced data packets including the pre-processed enhanced data and inserts known data to at least one of the enhanced data packets. A first multiplexer multiplexes the enhanced data packets with main data packets including the main data. And, an RS encoder RS-codes the multiplexed main and enhanced data packets, the RS encoder adding systematic parity data to each main data packet and adding RS parity plate holders to each enhanced data packet. Herein, the RS encoder may insert non-systematic RS parity data or null data into the RS parity place holders included in each enhanced data packet.