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This paper proposes an OFDM based transmission scheme for asynchronous amplify-and-forward cooperative systems with two relay nodes over frequency-selective channels. To combat the inter-symbol interference from multi-path fading and timing errors, the cyclic prefix (CP) is used at both the source node and relay nodes. Unlike using IDFT and DFT modulation for two consecutive OFDM blocks respectively, all data blocks are modulated by IDFT at the source node and demodulated by DFT at the destination node in the proposed scheme. A subcarrier basis symbol-by-symbol ML decoding can be employed by exploiting the Alamouti code structure of two consecutive OFDM blocks at the destination node. Simulation results show that the proposed scheme can achieve diversity order 2 over frequency-selective channels in the presence of asynchronism between nodes. Moreover, we prove that the proposed scheme over frequency selective fading channel has the identical BER performance with the case of flat fading. Extension to more relay nodes is straightforward and the proposed scheme can achieve diversity order equal to the number of relay nodes.