Physical layer security is studied for a wireless full duplex relay network, where relay selection scheme is adopted to enhance security performance. More specifically, considering the effect of inter-relay interference and residual self-interference, a Markov matrix is utilized to model the state transition probability for the number of successful decoding relays. Based on outdated channel state information between the relays and the legitimate destination, the secrecy outage probability under Rayleigh fading is investigated and its analytical expression is derived. The derived theoretical results are validated by computer simulations. The results reveal that our analytical results using Markov chain match the simulations, while the analytical method based only on probability theory cannot accurately predict the secrecy outage performance.