A receiving system and a method of processing broadcast signal are disclosed herein. The receiving system includes a signal receiving unit, an equalizer, an inner decoder, a reliability adjustor, and an outer decoder. The signal receiving unit receives a broadcast signal and demodulates the received broadcast signal, wherein the broadcast signal comprises a data group including mobile service data and multiple known data sequences. The equalizer uses the known data sequences to channel-equalize data group included in the demodulated broadcast signal. The inner decoder matches mobile service data within the data group, which are inputted in a block-size for turbo-decoding, with mobile service data being symbol-decoded and fed-back, thereby performing trellis-decoding. The reliability adjustor adjusts a reliability level of the mobile service data based upon positions within the data group of the trellis-decoded mobile service data. The outer decoder performs symbol-decoding on the mobile service data outputted from the reliability adjustor.