In this paper, we propose a robust symbol timing synchronization method for IEEE 802.11n wireless LAN (WLAN) systems. Symbol timing synchronization using a preamble is affected by the timing of the packet detection and automatic gain control (AGC) completion. However, conventional methods assumed that symbol timing synchronization starts at the beginning of the preamble, which is typically invalid in 802.11n systems. Also, the cyclic shift diversity (CSD) scheme is adopted for achieving spatial diversity and avoiding undesired beamforming in 802.11n standard. However, it induces a pseudo multipath problem which generates multiple-peaks of the cross-correlation. In order to solve these problems, a novel symbol timing synchronization method which is not affected by the pseudo multipath problem is devised. This method estimates a coarse time-offset of short training symbols (STSs) using the cross-correlation and detects a coarse boundary between the last STS and GI which is prior to long training symbols (LTSs) using maximum normalized correlation (MNC)-based algorithm and dividing the preamble into units of STS length. At last, to improve precision of boundary detection, the fine timing of GI start is refined by a verification stage using periodicity of STS. Simulation results show that the proposed method has the SNR gain of 2.4dB and 3.2 dB compared to the conventional method at the rate of 1% synchronization failure probability in 2TX-2RX and 4TX-4RX WLAN systems, respectively.