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Multiple input multiple output (MIMO) system has been recognized as a promising candidate for future wireless communication. The adaptive modulation which adjusts the transmitter parameters, such as modulation order, transmit power or coding rate, to time-varying channel conditions has been applied to MIMO system and shown a good average spectral efficiency performance. In this paper, the channel estimation error (CEE)'s effect on the effective spectral efficiency of the MIMO system with zero-forcing receiver is investigated, when the transmitter adopts adaptive modulation. To reduce CEE's negative effect, a dynamic adaptive modulation scheme is proposed. This scheme can dynamically adjust the signal to noise ratio (SNR) thresholds for the different modulation orders according to the feedbacks from the receiver. The numerical results show that the system performance of the proposed scheme is near optimal with acceptable implementation complexity.