In this paper, we propose a new low-density parity check (LDPC) coded bit mapping scheme for multiple-input multiple-output (MIMO) spatial multiplexing system. First, in order to enhance the decoding performance of LDPC code in MIMO system, we propose a new sub-channel mapping scheme which assigns systematic and parity bits to different MIMO sub-channel. Next, we propose the modified sub-channel mapping scheme to provide the additional coding gain by taking into consideration of the unequal protection (UEP) level of high order constellations. The simulation results show that the coding gain is enhanced by applying proposed mapping scheme on the MIMO spatial multiplexing systems.