A digital broadcast transmitter/receiver, and a signal processing method thereof, includes a randomizer randomizing a dual transport stream which includes a normal data packet and a robust data packet and into which stuff bytes are inserted, a stuff-byte exchanger replacing the stuff bytes of the randomized data with known data, a first RS encoder performing RS-encoding of data output from the stuff-byte exchanger, a packet formatter performing an interleaving of the robust packet of the data output from the first RS encoder and reformatting the packet, an interleaver interleaving data outputfrom the packet formatter, a trellis encoder performing a trellis encoding of interleaved data, a second RS encoder changing a parity by performing an RS encoding of the robust data of the trellis-encoded data, and a modulator modulating data output from the trellis encoder and RF up-converting the modulated data.