A method for transmitting data by a transmitter in a mobile communication system, comprising the steps of: encoding a data bit stream by an encoder; if number of the coded bits is not equal in number of bits transmitted from a radio channel, rate matching coded bits outputted from the encoder; outputting bits to be interleaved from the rate matched bit stream; inputting first bits from among the bits in the rate matched bit stream to a first interleave unit in two bits unit; inputting second bits from among the bits in the rate matched bit stream to a second interleave unit in the two bits unit; interleaving, by the first interleave unit, the first bits to output third bits in the two bits unit; interleaving, by a second interleave unit, the second bits to output fourth bits in the two bits unit; and generating, by a modulator, a modulation symbol with the third bits outputted from the first interleave unit and the fourth bits outputted from the second interleave unit, using a 16-ary quadrature amplitude modulation, 16QAM, wherein the first bits are systematic bits, and the second bits are mixed bits including systematic bits and parity bits, and the systematic bits, having higher priority relative to the parity bits in the mixed bits, are located in a front position of the respective two bits unit before interleaving, wherein the third bits are symbol-mapped to bit positions having higher reliability relative to bit positions to which the fourth bits are symbol-mapped, based on a predetermined symbol pattern by the modulator, and wherein position information of the systematic bits in the mixed bits before interleaving is pre-notified to a receiver by the transmitter. encoding a data bit stream by an encoder; if number of the coded bits is not equal in number of bits transmitted from a radio channel, rate matching coded bits outputted from the encoder; outputting bits to be interleaved from the rate matched bit stream; inputting first bits from among the bits in the rate matched bit stream to a first interleave unit in two bits unit; inputting second bits from among the bits in the rate matched bit stream to a second interleave unit in the two bits unit; interleaving, by the first interleave unit, the first bits to output third bits in the two bits unit; interleaving, by a second interleave unit, the second bits to output fourth bits in the two bits unit; and generating, by a modulator, a modulation symbol with the third bits outputted from the first interleave unit and the fourth bits outputted from the second interleave unit, using a 16-ary quadrature amplitude modulation, 16QAM, wherein the first bits are systematic bits, and the second bits are mixed bits including systematic bits and parity bits, and the systematic bits, having higher priority relative to the parity bits in the mixed bits, are located in a front position of the respective two bits unit before interleaving, wherein the third bits are symbol-mapped to bit positions having higher reliability relative to bit positions to which the fourth bits are symbol-mapped, based on a predetermined symbol pattern by the modulator, and wherein position information of the systematic bits in the mixed bits before interleaving is pre-notified to a receiver by the transmitter.