Full duplex transmission is a promising technique to enhance the capacity of relay communications. In this paper, we investigate the optimum transmit power allocation for full duplex decode-and-forward (DF) relaying systems. The outage probability is adopted as the optimization criterion and a transmit power optimization problem is formulated for systems with and without total power constraint. Analytical and numerical results are provided to show the advantages of the power optimization. More interestingly, the effect of residual self-interference introduced by full duplex transmission is examined in terms of optimal power allocation and outage probability performance improvement.