A method and apparatus for transmitting a broadcast signal in a transmitter are disclosed. The apparatus includes an RS encoder 212 configured to encode mobile data for forward error correction (FEC), a group formatter 214 configured to form a data group using (or including) the encoded mobile data and known data sequence, a packet formatter 216 configured to form a specified number of mobile data packets and a first scalable number of mobile data packets by using the data included in the data group, a packet multiplexer (MUX) 120 configured to multiplex the formed mobile data packets and a second scalable number of main data packets, a data interleaver 1502 configured to interleave data of the multiplexed data packets, and a transmission unit 170 configured to transmit the interleaved data. Herein, the known data are spotted in the interleaved data.