Abstract: The basic idea of cooperative communication is to obtain cooperative diversity by proper cooperation among different nodes to enhance the reliability of the communication. Employing Distributed Space Time Coding (DSTBC) at relay nodes, cooperative networks can achieve high bandwidth efficiency along with the full cooperative diversity. However, the time offsets among the relay nodes and multipath fading between nodes will destroy the structure of DSTBC, which may drastically undermine the diversity of cooperative networks. To mitigate the effects of symbol asynchronism in cooperative systems, OFDM technique can be used in DSTBC based asynchronous cooperative systems, which can obtain full space diversity with the existence of timing errors. But the existing schemes are based on the assumption of flat fading channels between nodes and they could not extend to multipath channels straightforwardly. In addition, the DSTBC required in these schemes should satisfy some constraints. In this paper, a novel OFDM based transmission scheme for asynchronous cooperative systems is proposed, which can achieve full space diversity over multipath channels. Moreover, all DSTBCs for synchronous cooperative systems can be applied to the proposed scheme. The analysis and simulation results demonstrate that the proposed scheme over multipath channels has same BER performance with the existing OFDM based scheme over flat fading channels. Furthermore, for the systems with two relays, we employ subcarrier grouping and precoding at the source node in the proposed scheme to achieve full space diversity along with full multipath diversity.