Abstract: One of the most important issues of pulse position modulation (PPM) for optical wireless communications is the receiver symbol synchronization subsystem, in this paper, the basic principle of symbol synchronization for PPM sequence is discussed, and the upper limit of exact synchronization probability for random PPM sequence is analyzed. To solve this problem a new scheme of inserting synchronous back-to-back PPM symbols to the PPM sequence at intervals of certain symbols is presented, the synchronization signal is obtained by searching for the synchronous symbols, and the corresponding digital logic circuit is presented too. Theory and experiment results show that the exact PPM symbol synchronization can be obtained by this scheme with more credibility compared to the scheme proposed by Ref.8.