In this letter, a robust time synchronization algorithm is proposed for MIMO-OFDM based WLAN systems. IEEE 802.11ac MIMO-OFDM WLAN standard specifies that the preamble with cyclic shift diversity (CSD) scheme is used for time and frequency synchronization. However, since the CSD scheme introduces multiple cross-correlation peaks at the receiver, serious performance degradation appears if the conventional cross-correlation based algorithm is applied. In the proposed algorithm, the time synchronization error due to multiple peaks is compensated by adding the cross-correlation value to its reverse cyclic-shifted version. Simulation results show that the proposed algorithm achieves an SNR gain of 1.5 to 4.5dB for the synchronization failure rate of 10-2 compared with the existing algorithms.