In this letter, we analyze the bit error rate (BER) of a multiple-input multiple-output bit-interleaved coded modulation (MIMO-BICM) system with an iterative receiver that consists of a soft-in soft-out (SISO) minimum mean square error detector and an SISO channel decoder. At a current detection/decoding cycle, an upper bound on a BER is derived based on the moment generating function of signal-to-interference-and-noise ratio at the detector output. We utilize the analytical BER derived from the current cycle to update the upper bound at the next cycle in an iterative manner. By an asymptotic approach, we also show that the BER of the MIMO-BICM system with an iterative receiver converges to that of the maximal ratio combining system.