A new preamble structure is designed for wireless LAN based on MIMO OFDM systems, which can be used for both synchronization and channel estimation. Modulatable orthogonal polyphase sequence is utilized in training symbol design regarding its correlation properties. The time synchronization and channel estimation are achieved by measuring the correlation between the Received training sequence and the locally generated training sequence. Repeated training symbols are used to get carrier frequency offset(CFO)estimation. It is shown from the analysis that the accuracy of frequency synchronization is close to the Cram6r-Rao lower bound.The training sequences are optimal for channel estimation based on the minimum mean square error(MMSE).