In this paper, new training sequences and frame structure are proposed to estimate time-varying channel for MIMO OFDM systems. The training sequences named modulatable orthogonal polyphase sequences have both good autocorrelations and cross-correlations. They are used as guard interval instead of cyclic prefix, which not only improve the transmission efficiency but also make the channel estimator can track time-varying channel. The channel impulse response can be obtained by measuring the correlation between the received training sequence and the locally generated training sequence. The simulation results show that the proposed estimation method has better performance than conventional methods in fast time-varying channel.