A DTV transmitter includes a pre-processor which pre-processes enhanced data by coding the enhanced data for forward error correction (FEC) and expanding the FEC-coded enhanced data, a data formatter which generates enhanced data packets having the pre-processed enhanced data and known data, and a multiplexer which multiplexes the enhanced data packets with main data packets. The DTV transmitter further includes an RS encoder which adds systematic parity data to each main data packet and adds RS parity place holders to each enhanced data packet, and a data interleaver which interleaves the RS-coded main and enhanced data packets and outputs a group of interleaved data packets having a head, a body, and a tail. The body includes a plurality of consecutive enhanced data packets, to which a known data sequence is periodically inserted.