Distributed space-time-frequency coding (DSTFC) is a method to achieve diversity from both multipaths and user cooperations. The DSTFC has been researched by assuming that the multipath profiles of relays are the same (symmetric). However, the symmetric frequency- selective channel is limited in the wireless relay communications. Thus, in this paper, we propose a new DSTFC for relay-based amplify-and-forward (AF) cooperative scheme over the asymmetric frequency selective channels. The proposed scheme is based on the sub-carrier grouping in OFDM transmission. By analyzing pairwise error probability (PEP), diversity order of the proposed DSTFC is derived. In addition, a necessary condition of sub-carrier grouping to achieve the maximum diversity and minimize the decoding complexity is investigated. Simulation results verify that the proposed scheme can achieve the maximum diversity gain for a given asymmetric multipath profile.