A preamble structure for orthogonal frequency division multiplexing（OFDM） based power line carrier communication system is designed in order to improve its ability to provide robust communication in hostile power line channel. Based on the structure， a new timing synchronization algorithm and a mode identification scheme are proposed. MATLAB simulation shows that the proposed timing synchronization approach can provide more competitive timing offset mean and variance than those of traditional methods even in extremely harsh power line channel. Different system operation modes can also be identified unambiguously in lower signal noise ratios with the proposed mode identification.