Abstract: Employing Distributed Space Time Coding (DSTC) 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 DSTC, which may drastically undermine the diversity potential of cooperative networks. This paper proposes a novel DSTC transmission scheme based on linear pre-processing for asynchronous cooperative systems with two relay nodes over frequency selective channels. In this proposed scheme, the data blocks are pre-processed at the source node and forwarded to relay nodes, and then permutation and conjugation operations are employed at the relay nodes to construct DSTC structure at the destination node. To combat the inter symbol interference (ISI) from multipath fading and timing errors, the cyclic prefix (CP) based transmission is considered at both the source node and the relay nodes. After DFT operations at the destination node, maximum likelihood (ML) detection can be applied to recover the transmitted signals. The analysis and simulation results demonstrate that the proposed scheme can achieve full space diversity and full multipath diversity for cooperative communication over frequency selective channels in the presence of asynchronism between relay nodes. Furthermore, the case that the time delay between multipath components is integral multiple symbol periods is considered in this paper. It is proved that the diversity order of the proposed scheme is only related with the effective channel length.