In this paper, the issue of channel estimation is investigated for distributed space-frequency coding system in cooperative communications. To make the channel being identifiable, Cyclic Convolution Filters (CCF) are utilized at the relay nodes. In the training stage, the suboptimal design is derived for both the pilot sequence and CCFs adopted at relay nodes. Furthermore, the closed expression of power allocation between source node and relay nodes is obtained. Finally, computer simulations demonstrate the effectiveness of the proposed approach.