The detection of V-BLAST system always requires an optimal ordering procedure, and also it requires knowledge of the channel state information (CSI) while in practice accurate CSI may not be available. In this paper, we investigate the system with two transmit antennas to evaluate its performance, considering both the effects of channel estimation error and optimal ordering procedure. Based on the modified postprocessing SNR, we derive the closed-form analytical BER expressions for MPSK and MQAM modulation at each processing stage. Monte Carlo simulations validate the analytical results and conclusions.