In this paper STBC is applied to IDMA systems (called STBC-IDMA systems). Performance comparison between STBC-IDMA and uncoded IDMA systems with different number of users, and performances of STBC-IDMA systems with different transmit antennas are investigated over quasi-static Rayleigh fading channel. Simulation results show that the system proposed in this paper owns the same throughput and acquires greater diversity gain contrast to the uncoded IDMA system at the bit error rate (BER) level of 10-3 and 10-5 with different number of users. Furthermore, it is found that the more transmit antennas, the more diversity gain in proposed systems.