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 output from 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.