In this paper, we propose a novel cooperative relaying scheme with multiple relays for frequency selective wireless environment. In our proposed scheme, the frequency selective wireless channel is divided into flat fading sub-channels. Cooperative relaying is then employed over each sub-channel to improve the system diversity order. We then investigate the asymptotic behavior of the outage probability and show that the proposed scheme achieves full diversity order in both amplify and forward (AF) and adaptive decode and forward (ADF) relaying scenarios. Furthermore, we propose a power allocation strategy to minimize the system outage probability. Simulation results confirm our analysis and show that the proposed power allocation method outperforms uniform power allocation.